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    SubjectRe: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs
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    On 10/17/2023 11:47 PM, John Allen wrote:
    > On Thu, Oct 12, 2023 at 02:31:19PM +0530, Nikunj A. Dadhania wrote:
    >> On 10/11/2023 1:32 AM, John Allen wrote:
    >>> If kvm supports shadow stack, pass through shadow stack MSRs to improve
    >>> guest performance.
    >>>
    >>> Signed-off-by: John Allen <john.allen@amd.com>
    >>> ---
    >>> arch/x86/kvm/svm/svm.c | 26 ++++++++++++++++++++++++++
    >>> arch/x86/kvm/svm/svm.h | 2 +-
    >>> 2 files changed, 27 insertions(+), 1 deletion(-)
    >>>
    >>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
    >>> index e435e4fbadda..984e89d7a734 100644
    >>> --- a/arch/x86/kvm/svm/svm.c
    >>> +++ b/arch/x86/kvm/svm/svm.c
    >>> @@ -139,6 +139,13 @@ static const struct svm_direct_access_msrs {
    >>> { .index = X2APIC_MSR(APIC_TMICT), .always = false },
    >>> { .index = X2APIC_MSR(APIC_TMCCT), .always = false },
    >>> { .index = X2APIC_MSR(APIC_TDCR), .always = false },
    >>> + { .index = MSR_IA32_U_CET, .always = false },
    >>> + { .index = MSR_IA32_S_CET, .always = false },
    >>> + { .index = MSR_IA32_INT_SSP_TAB, .always = false },
    >>> + { .index = MSR_IA32_PL0_SSP, .always = false },
    >>> + { .index = MSR_IA32_PL1_SSP, .always = false },
    >>> + { .index = MSR_IA32_PL2_SSP, .always = false },
    >>> + { .index = MSR_IA32_PL3_SSP, .always = false },
    >>
    >> First three MSRs are emulated in the patch 1, any specific reason for skipping MSR_IA32_PL[0-3]_SSP ?
    >
    > I'm not sure what you mean.

    MSR_IA32_U_CET, MSR_IA32_S_CET and MSR_IA32_INT_SSP_TAB are selectively emulated and there is no good explanation why MSR_IA32_PL[0-3]_SSP do not need emulation. Moreover, MSR interception is initially set (i.e. always=false) for MSR_IA32_PL[0-3]_SSP.

    > The PLx_SSP MSRS should be getting passed
    > through here unless I'm misunderstanding something.

    In that case, intercept should be cleared from the very beginning.

    + { .index = MSR_IA32_PL0_SSP, .always = true },
    + { .index = MSR_IA32_PL1_SSP, .always = true },
    + { .index = MSR_IA32_PL2_SSP, .always = true },
    + { .index = MSR_IA32_PL3_SSP, .always = true },

    Regards
    Nikunj


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