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path and unconditionally uses LSL to retrieve the CPU number. RDPID depends on MSR_TSX_AUX. KVM has an optmization to avoid expensive MRS read/writes on VMENTER/EXIT. It caches the MSR values and restores them either when leaving the run loop, on preemption or when going out to user space. MSR_TSX_AUX is part of that lazy MSR set, so after writing the guest value and before the lazy restore any exception using the paranoid entry will read the guest value and use it as CPU number to retrieve the GSBASE value for the current CPU when FSGSBASE is enabled. As RDPID is only used in that particular entry path, there is no reason to burden VMENTER/EXIT with two extra MSR writes. Remove the RDPID optimization, which is not even backed by numbers from the paranoid entry path instead. -----BEGIN PGP SIGNATURE----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAl9CJqgTHHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYoaAvD/sHkSx1V0hupUh9budLhSPhUyWvXiar AmAvWl8dKEFG2UOhOT57zD6DgFN3uu5rqjxTG7ha9XEspsVaP5jOji4evus37IAe z2EB9J5c3ih4VSdaM+8ODTMls5rjQrvJjeDV0ETTQ9Xb+FOT0vNAub6D5PVms6J3 FObDKiIpdB3s5INAWPknIYves0EJP5BP3+gOktovMStfcy8tm8N9S/yA4cNL4nbi IZ0h1H6xcGrQ79dv+2/vC0cdqlbm6y2KWfNKpSTGwPMdRl0PpHrovsOVPKF+6pio Ad230t7xnWMlrHghbSvDyyJ67/N6AA6CaqHECWtgsDuzbqcD2MoQ2l97atoZInh7 83n8ZWFaw10T4ksw9SWqAex+ZJh6/rD4vcQYUncPN66/kOVM186ezICc+QsPV99s ukw29xge4uHz91Hy0Bo8SP+w1bvntKJn6XyJuTFgDt8bmFRIeajSxyOGw7hTs+ZD TONw9dMeteWZhZRIXYDjlYc83xFYGkX6hmxLrDJ4jg8UGojaca83s7oZtxgZxWzu L9wfCRJIEA33ihvqtbTEOHbJvl6eyDt8b/kBGGHbAbqweQ3mWEH3WDQ5cyhlLrfA tnDToX1DvsxfVg94saprNt249qHNZlFIj8EaGfjxxEngd8xAgfU0vxJpI8sBtdsw SMIwyLLbLz573A== =AEel -----END PGP SIGNATURE----- Merge tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull x86 fix from Thomas Gleixner: "A single fix for x86 which removes the RDPID usage from the paranoid entry path and unconditionally uses LSL to retrieve the CPU number. RDPID depends on MSR_TSX_AUX. KVM has an optmization to avoid expensive MRS read/writes on VMENTER/EXIT. It caches the MSR values and restores them either when leaving the run loop, on preemption or when going out to user space. MSR_TSX_AUX is part of that lazy MSR set, so after writing the guest value and before the lazy restore any exception using the paranoid entry will read the guest value and use it as CPU number to retrieve the GSBASE value for the current CPU when FSGSBASE is enabled. As RDPID is only used in that particular entry path, there is no reason to burden VMENTER/EXIT with two extra MSR writes. Remove the RDPID optimization, which is not even backed by numbers from the paranoid entry path instead" * tag 'x86-urgent-2020-08-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.