linux/arch/x86/kvm/svm/svm_ops.h

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/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __KVM_X86_SVM_OPS_H
#define __KVM_X86_SVM_OPS_H
#include <linux/compiler_types.h>
#include "x86.h"
#define svm_asm(insn, clobber...) \
do { \
asm_volatile_goto("1: " __stringify(insn) "\n\t" \
_ASM_EXTABLE(1b, %l[fault]) \
::: clobber : fault); \
return; \
fault: \
kvm_spurious_fault(); \
} while (0)
#define svm_asm1(insn, op1, clobber...) \
do { \
asm_volatile_goto("1: " __stringify(insn) " %0\n\t" \
_ASM_EXTABLE(1b, %l[fault]) \
:: op1 : clobber : fault); \
return; \
fault: \
kvm_spurious_fault(); \
} while (0)
#define svm_asm2(insn, op1, op2, clobber...) \
do { \
asm_volatile_goto("1: " __stringify(insn) " %1, %0\n\t" \
_ASM_EXTABLE(1b, %l[fault]) \
:: op1, op2 : clobber : fault); \
return; \
fault: \
kvm_spurious_fault(); \
} while (0)
static inline void clgi(void)
{
svm_asm(clgi);
}
static inline void stgi(void)
{
svm_asm(stgi);
}
static inline void invlpga(unsigned long addr, u32 asid)
{
svm_asm2(invlpga, "c"(asid), "a"(addr));
}
/*
* Despite being a physical address, the portion of rAX that is consumed by
* VMSAVE, VMLOAD, etc... is still controlled by the effective address size,
* hence 'unsigned long' instead of 'hpa_t'.
*/
static __always_inline void vmsave(unsigned long pa)
{
svm_asm1(vmsave, "a" (pa), "memory");
}
static __always_inline void vmload(unsigned long pa)
KVM: SVM: use vmsave/vmload for saving/restoring additional host state Using a guest workload which simply issues 'hlt' in a tight loop to generate VMEXITs, it was observed (on a recent EPYC processor) that a significant amount of the VMEXIT overhead measured on the host was the result of MSR reads/writes in svm_vcpu_load/svm_vcpu_put according to perf: 67.49%--kvm_arch_vcpu_ioctl_run | |--23.13%--vcpu_put | kvm_arch_vcpu_put | | | |--21.31%--native_write_msr | | | --1.27%--svm_set_cr4 | |--16.11%--vcpu_load | | | --15.58%--kvm_arch_vcpu_load | | | |--13.97%--svm_set_cr4 | | | | | |--12.64%--native_read_msr Most of these MSRs relate to 'syscall'/'sysenter' and segment bases, and can be saved/restored using 'vmsave'/'vmload' instructions rather than explicit MSR reads/writes. In doing so there is a significant reduction in the svm_vcpu_load/svm_vcpu_put overhead measured for the above workload: 50.92%--kvm_arch_vcpu_ioctl_run | |--19.28%--disable_nmi_singlestep | |--13.68%--vcpu_load | kvm_arch_vcpu_load | | | |--9.19%--svm_set_cr4 | | | | | --6.44%--native_read_msr | | | --3.55%--native_write_msr | |--6.05%--kvm_inject_nmi |--2.80%--kvm_sev_es_mmio_read |--2.19%--vcpu_put | | | --1.25%--kvm_arch_vcpu_put | native_write_msr Quantifying this further, if we look at the raw cycle counts for a normal iteration of the above workload (according to 'rdtscp'), kvm_arch_vcpu_ioctl_run() takes ~4600 cycles from start to finish with the current behavior. Using 'vmsave'/'vmload', this is reduced to ~2800 cycles, a savings of 39%. While this approach doesn't seem to manifest in any noticeable improvement for more realistic workloads like UnixBench, netperf, and kernel builds, likely due to their exit paths generally involving IO with comparatively high latencies, it does improve overall overhead of KVM_RUN significantly, which may still be noticeable for certain situations. It also simplifies some aspects of the code. With this change, explicit save/restore is no longer needed for the following host MSRs, since they are documented[1] as being part of the VMCB State Save Area: MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK, MSR_KERNEL_GS_BASE, MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP, MSR_FS_BASE, MSR_GS_BASE and only the following MSR needs individual handling in svm_vcpu_put/svm_vcpu_load: MSR_TSC_AUX We could drop the host_save_user_msrs array/loop and instead handle MSR read/write of MSR_TSC_AUX directly, but we leave that for now as a potential follow-up. Since 'vmsave'/'vmload' also handles the LDTR and FS/GS segment registers (and associated hidden state)[2], some of the code previously used to handle this is no longer needed, so we drop it as well. The first public release of the SVM spec[3] also documents the same handling for the host state in question, so we make these changes unconditionally. Also worth noting is that we 'vmsave' to the same page that is subsequently used by 'vmrun' to record some host additional state. This is okay, since, in accordance with the spec[2], the additional state written to the page by 'vmrun' does not overwrite any fields written by 'vmsave'. This has also been confirmed through testing (for the above CPU, at least). [1] AMD64 Architecture Programmer's Manual, Rev 3.33, Volume 2, Appendix B, Table B-2 [2] AMD64 Architecture Programmer's Manual, Rev 3.31, Volume 3, Chapter 4, VMSAVE/VMLOAD [3] Secure Virtual Machine Architecture Reference Manual, Rev 3.01 Suggested-by: Tom Lendacky <thomas.lendacky@amd.com> Signed-off-by: Michael Roth <michael.roth@amd.com> Message-Id: <20210202190126.2185715-2-michael.roth@amd.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2021-02-03 03:01:24 +08:00
{
svm_asm1(vmload, "a" (pa), "memory");
}
#endif /* __KVM_X86_SVM_OPS_H */