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    SubjectRe: [PATCH v19 086/130] KVM: TDX: restore debug store when TD exit
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    Hi Isaku,

    On 2/26/2024 12:26 AM, isaku.yamahata@intel.com wrote:
    > From: Isaku Yamahata <isaku.yamahata@intel.com>
    >
    > Because debug store is clobbered, restore it on TD exit.
    >

    I am trying to understand why this is needed and finding a few
    places that seem to indicate that this is not needed.

    For example, in the TX base spec, section 15.2 "On-TD Performance
    Monitoring", subsection "15.2.1 Overview":
    * IA32_DS_AREA MSR is context-switched across TD entry and exit transitions.

    To confirm I peeked at the TDX module code and found (if I understand this
    correctly) that IA32_DS_AREA is saved on TD entry (see [1]) and restored on
    TD exit (see [2]).


    > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
    > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
    > ---
    > arch/x86/events/intel/ds.c | 1 +
    > arch/x86/kvm/vmx/tdx.c | 1 +
    > 2 files changed, 2 insertions(+)
    >
    > diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
    > index d49d661ec0a7..25670d8a485b 100644
    > --- a/arch/x86/events/intel/ds.c
    > +++ b/arch/x86/events/intel/ds.c
    > @@ -2428,3 +2428,4 @@ void perf_restore_debug_store(void)
    >
    > wrmsrl(MSR_IA32_DS_AREA, (unsigned long)ds);
    > }
    > +EXPORT_SYMBOL_GPL(perf_restore_debug_store);
    > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
    > index b8b168f74dfe..ad4d3d4eaf6c 100644
    > --- a/arch/x86/kvm/vmx/tdx.c
    > +++ b/arch/x86/kvm/vmx/tdx.c
    > @@ -665,6 +665,7 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
    > tdx_vcpu_enter_exit(tdx);
    >
    > tdx_user_return_update_cache(vcpu);
    > + perf_restore_debug_store();
    > tdx_restore_host_xsave_state(vcpu);
    > tdx->host_state_need_restore = true;
    >

    Reinette

    [1] https://github.com/intel/tdx-module/blob/tdx_1.5/src/td_transitions/tdh_vp_enter.c#L719
    [2] https://github.com/intel/tdx-module/blob/tdx_1.5/src/td_transitions/td_exit.c#L130

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