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* Ratelimit the message about writes to unrecognized MSRs so that they
don't spam the console log, by Chris Down. * Document how the /proc/cpuinfo machinery works for future reference, by Kyung Min Park, Ricardo Neri and Dave Hansen. * Correct the current NMI's duration calculation, by Libing Zhou. -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEEzv7L6UO9uDPlPSfHEsHwGGHeVUoFAl+ELfcACgkQEsHwGGHe VUoZ4g//V5dvkmniFs/ZWvFmHvCqdtovQgT0X7C3iHxwdGorrP/2Y7+Ocey4Nss0 TmpQXMnrvxSniMBCSh5Fodm9w1th+5IA6lJ/OETza3ANJyGHswQ9P5FfcY0QtGiE FUPapV3HKY6G3HGDXDLokCPLeQduY1MeKSRzKAw7E+735+TPlZcmCiTqPBRWcBfw PWNo9eOJuJMnl+7XbCqCBb7Z8o+hb0jqD42Ky6IL9sVMEVT+UtH50kkf0uJciMjl 23ga1LIPMYJ3vw0mPm945ixNpqxkDsYBQx5Q3PyGHAgqWq9TFytffLY3AScGrUpx EuzOLWmN2zjAEY2Yw454a9fbKj1b2d4M48JmNyKdG0KxH1X9rtoeS4ECJrXAjd0e BukrieymwSR6CHj4Yp8ZAq0XVtp7AUdeEOWCMTezbr5Fj+C4GWPetHXurM5C5Gj5 Xes0lkA+wFVzPL6R0gkUBsVXVxtZNmMVfn9HKj7bQP8Ar00g5osZ2yFHduXw8BzA blYowJ2mt54Pb1ertOM3zxKgIqj0PV2vSz8lRPyXzd4xdkkg6WxzP1EtDj5qHARf +1X2jBWWY1rNosQi2NlzZRO+GiBOPevyVfxZ1KgRkWRtP9hED8kdfVkhLqPBEP6W DvnKSAAVQiQe+6HP/anNDKy3TzjX5B3/ZgbZDCt1x78swjLXJwE= =1bX/ -----END PGP SIGNATURE----- Merge tag 'x86_misc_for_v5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull misc x86 fixes fromm Borislav Petkov: - Ratelimit the message about writes to unrecognized MSRs so that they don't spam the console log (Chris Down) - Document how the /proc/cpuinfo machinery works for future reference (Kyung Min Park, Ricardo Neri and Dave Hansen) - Correct the current NMI's duration calculation (Libing Zhou) * tag 'x86_misc_for_v5.10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/nmi: Fix nmi_handle() duration miscalculation Documentation/x86: Add documentation for /proc/cpuinfo feature flags x86/msr: Make source of unrecognised MSR writes unambiguous x86/msr: Prevent userspace MSR access from dominating the console
This commit is contained in:
Linus Torvalds
commit
9e536c8179
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Documentation/x86/cpuinfo.rst
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Documentation/x86/cpuinfo.rst
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.. SPDX-License-Identifier: GPL-2.0
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=================
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x86 Feature Flags
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=================
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Introduction
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============
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On x86, flags appearing in /proc/cpuinfo have an X86_FEATURE definition
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in arch/x86/include/asm/cpufeatures.h. If the kernel cares about a feature
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or KVM want to expose the feature to a KVM guest, it can and should have
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an X86_FEATURE_* defined. These flags represent hardware features as
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well as software features.
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If users want to know if a feature is available on a given system, they
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try to find the flag in /proc/cpuinfo. If a given flag is present, it
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means that the kernel supports it and is currently making it available.
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If such flag represents a hardware feature, it also means that the
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hardware supports it.
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If the expected flag does not appear in /proc/cpuinfo, things are murkier.
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Users need to find out the reason why the flag is missing and find the way
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how to enable it, which is not always easy. There are several factors that
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can explain missing flags: the expected feature failed to enable, the feature
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is missing in hardware, platform firmware did not enable it, the feature is
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disabled at build or run time, an old kernel is in use, or the kernel does
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not support the feature and thus has not enabled it. In general, /proc/cpuinfo
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shows features which the kernel supports. For a full list of CPUID flags
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which the CPU supports, use tools/arch/x86/kcpuid.
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How are feature flags created?
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==============================
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a: Feature flags can be derived from the contents of CPUID leaves.
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------------------------------------------------------------------
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These feature definitions are organized mirroring the layout of CPUID
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leaves and grouped in words with offsets as mapped in enum cpuid_leafs
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in cpufeatures.h (see arch/x86/include/asm/cpufeatures.h for details).
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If a feature is defined with a X86_FEATURE_<name> definition in
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cpufeatures.h, and if it is detected at run time, the flags will be
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displayed accordingly in /proc/cpuinfo. For example, the flag "avx2"
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comes from X86_FEATURE_AVX2 in cpufeatures.h.
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b: Flags can be from scattered CPUID-based features.
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----------------------------------------------------
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Hardware features enumerated in sparsely populated CPUID leaves get
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software-defined values. Still, CPUID needs to be queried to determine
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if a given feature is present. This is done in init_scattered_cpuid_features().
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For instance, X86_FEATURE_CQM_LLC is defined as 11*32 + 0 and its presence is
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checked at runtime in the respective CPUID leaf [EAX=f, ECX=0] bit EDX[1].
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The intent of scattering CPUID leaves is to not bloat struct
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cpuinfo_x86.x86_capability[] unnecessarily. For instance, the CPUID leaf
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[EAX=7, ECX=0] has 30 features and is dense, but the CPUID leaf [EAX=7, EAX=1]
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has only one feature and would waste 31 bits of space in the x86_capability[]
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array. Since there is a struct cpuinfo_x86 for each possible CPU, the wasted
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memory is not trivial.
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c: Flags can be created synthetically under certain conditions for hardware features.
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-------------------------------------------------------------------------------------
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Examples of conditions include whether certain features are present in
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MSR_IA32_CORE_CAPS or specific CPU models are identified. If the needed
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conditions are met, the features are enabled by the set_cpu_cap or
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setup_force_cpu_cap macros. For example, if bit 5 is set in MSR_IA32_CORE_CAPS,
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the feature X86_FEATURE_SPLIT_LOCK_DETECT will be enabled and
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"split_lock_detect" will be displayed. The flag "ring3mwait" will be
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displayed only when running on INTEL_FAM6_XEON_PHI_[KNL|KNM] processors.
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d: Flags can represent purely software features.
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------------------------------------------------
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These flags do not represent hardware features. Instead, they represent a
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software feature implemented in the kernel. For example, Kernel Page Table
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Isolation is purely software feature and its feature flag X86_FEATURE_PTI is
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also defined in cpufeatures.h.
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Naming of Flags
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===============
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The script arch/x86/kernel/cpu/mkcapflags.sh processes the
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#define X86_FEATURE_<name> from cpufeatures.h and generates the
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x86_cap/bug_flags[] arrays in kernel/cpu/capflags.c. The names in the
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resulting x86_cap/bug_flags[] are used to populate /proc/cpuinfo. The naming
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of flags in the x86_cap/bug_flags[] are as follows:
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a: The name of the flag is from the string in X86_FEATURE_<name> by default.
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----------------------------------------------------------------------------
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By default, the flag <name> in /proc/cpuinfo is extracted from the respective
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X86_FEATURE_<name> in cpufeatures.h. For example, the flag "avx2" is from
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X86_FEATURE_AVX2.
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b: The naming can be overridden.
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--------------------------------
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If the comment on the line for the #define X86_FEATURE_* starts with a
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double-quote character (""), the string inside the double-quote characters
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will be the name of the flags. For example, the flag "sse4_1" comes from
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the comment "sse4_1" following the X86_FEATURE_XMM4_1 definition.
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There are situations in which overriding the displayed name of the flag is
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needed. For instance, /proc/cpuinfo is a userspace interface and must remain
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constant. If, for some reason, the naming of X86_FEATURE_<name> changes, one
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shall override the new naming with the name already used in /proc/cpuinfo.
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c: The naming override can be "", which means it will not appear in /proc/cpuinfo.
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----------------------------------------------------------------------------------
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The feature shall be omitted from /proc/cpuinfo if it does not make sense for
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the feature to be exposed to userspace. For example, X86_FEATURE_ALWAYS is
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defined in cpufeatures.h but that flag is an internal kernel feature used
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in the alternative runtime patching functionality. So, its name is overridden
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with "". Its flag will not appear in /proc/cpuinfo.
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Flags are missing when one or more of these happen
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==================================================
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a: The hardware does not enumerate support for it.
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--------------------------------------------------
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For example, when a new kernel is running on old hardware or the feature is
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not enabled by boot firmware. Even if the hardware is new, there might be a
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problem enabling the feature at run time, the flag will not be displayed.
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b: The kernel does not know about the flag.
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-------------------------------------------
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For example, when an old kernel is running on new hardware.
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c: The kernel disabled support for it at compile-time.
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------------------------------------------------------
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For example, if 5-level-paging is not enabled when building (i.e.,
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CONFIG_X86_5LEVEL is not selected) the flag "la57" will not show up [#f1]_.
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Even though the feature will still be detected via CPUID, the kernel disables
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it by clearing via setup_clear_cpu_cap(X86_FEATURE_LA57).
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d: The feature is disabled at boot-time.
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----------------------------------------
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A feature can be disabled either using a command-line parameter or because
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it failed to be enabled. The command-line parameter clearcpuid= can be used
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to disable features using the feature number as defined in
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/arch/x86/include/asm/cpufeatures.h. For instance, User Mode Instruction
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Protection can be disabled using clearcpuid=514. The number 514 is calculated
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from #define X86_FEATURE_UMIP (16*32 + 2).
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In addition, there exists a variety of custom command-line parameters that
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disable specific features. The list of parameters includes, but is not limited
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to, nofsgsbase, nosmap, and nosmep. 5-level paging can also be disabled using
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"no5lvl". SMAP and SMEP are disabled with the aforementioned parameters,
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respectively.
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e: The feature was known to be non-functional.
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----------------------------------------------
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The feature was known to be non-functional because a dependency was
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missing at runtime. For example, AVX flags will not show up if XSAVE feature
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is disabled since they depend on XSAVE feature. Another example would be broken
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CPUs and them missing microcode patches. Due to that, the kernel decides not to
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enable a feature.
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.. [#f1] 5-level paging uses linear address of 57 bits.
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@ -9,6 +9,7 @@ x86-specific Documentation
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:numbered:
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boot
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cpuinfo
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topology
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exception-tables
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kernel-stacks
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@ -80,18 +80,30 @@ static ssize_t msr_read(struct file *file, char __user *buf,
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static int filter_write(u32 reg)
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{
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/*
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* MSRs writes usually happen all at once, and can easily saturate kmsg.
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* Only allow one message every 30 seconds.
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*
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* It's possible to be smarter here and do it (for example) per-MSR, but
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* it would certainly be more complex, and this is enough at least to
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* avoid saturating the ring buffer.
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*/
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static DEFINE_RATELIMIT_STATE(fw_rs, 30 * HZ, 1);
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switch (allow_writes) {
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case MSR_WRITES_ON: return 0;
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case MSR_WRITES_OFF: return -EPERM;
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default: break;
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}
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if (!__ratelimit(&fw_rs))
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return 0;
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if (reg == MSR_IA32_ENERGY_PERF_BIAS)
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return 0;
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pr_err_ratelimited("Write to unrecognized MSR 0x%x by %s\n"
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"Please report to x86@kernel.org\n",
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reg, current->comm);
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pr_err("Write to unrecognized MSR 0x%x by %s (pid: %d). Please report to x86@kernel.org.\n",
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reg, current->comm, current->pid);
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return 0;
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}
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@ -102,7 +102,6 @@ fs_initcall(nmi_warning_debugfs);
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static void nmi_check_duration(struct nmiaction *action, u64 duration)
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{
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u64 whole_msecs = READ_ONCE(action->max_duration);
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int remainder_ns, decimal_msecs;
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if (duration < nmi_longest_ns || duration < action->max_duration)
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@ -110,12 +109,12 @@ static void nmi_check_duration(struct nmiaction *action, u64 duration)
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action->max_duration = duration;
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remainder_ns = do_div(whole_msecs, (1000 * 1000));
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remainder_ns = do_div(duration, (1000 * 1000));
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decimal_msecs = remainder_ns / 1000;
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printk_ratelimited(KERN_INFO
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"INFO: NMI handler (%ps) took too long to run: %lld.%03d msecs\n",
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action->handler, whole_msecs, decimal_msecs);
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action->handler, duration, decimal_msecs);
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}
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static int nmi_handle(unsigned int type, struct pt_regs *regs)
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