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    SubjectRe: [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of IA32_XFD_ERR
    On Wed, Dec 29, 2021, Yang Zhong wrote:
    > From: Jing Liu <jing2.liu@intel.com>
    >
    > Disable read emulation of IA32_XFD_ERR MSR if guest cpuid includes XFD.
    > This saves one unnecessary VM-exit in guest #NM handler, given that the
    > MSR is already restored with the guest value before the guest is resumed.
    >
    > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
    > Signed-off-by: Jing Liu <jing2.liu@intel.com>
    > Signed-off-by: Yang Zhong <yang.zhong@intel.com>
    > ---
    > arch/x86/kvm/vmx/vmx.c | 6 ++++++
    > arch/x86/kvm/vmx/vmx.h | 2 +-
    > 2 files changed, 7 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    > index 4e51de876085..638665b3e241 100644
    > --- a/arch/x86/kvm/vmx/vmx.c
    > +++ b/arch/x86/kvm/vmx/vmx.c
    > @@ -162,6 +162,7 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
    > MSR_FS_BASE,
    > MSR_GS_BASE,
    > MSR_KERNEL_GS_BASE,
    > + MSR_IA32_XFD_ERR,
    > #endif
    > MSR_IA32_SYSENTER_CS,
    > MSR_IA32_SYSENTER_ESP,
    > @@ -7228,6 +7229,11 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
    > }
    > }
    >
    > + if (boot_cpu_has(X86_FEATURE_XFD))

    This should be kvm_cpu_cap_has(), not boot_cpu_has(). If 32-bit kernels don't
    suppress XFD in boot_cpu_data, then using boot_cpus_has() is wrong. And even if
    XFD is suppressed, using kvm_cpu_cap_has() is still preferable.

    > + vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R,
    > + !guest_cpuid_has(vcpu, X86_FEATURE_XFD));
    > +
    > +
    > set_cr4_guest_host_mask(vmx);
    >
    > vmx_write_encls_bitmap(vcpu, NULL);
    > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
    > index 4df2ac24ffc1..bf9d3051cd6c 100644
    > --- a/arch/x86/kvm/vmx/vmx.h
    > +++ b/arch/x86/kvm/vmx/vmx.h
    > @@ -340,7 +340,7 @@ struct vcpu_vmx {
    > struct lbr_desc lbr_desc;
    >
    > /* Save desired MSR intercept (read: pass-through) state */
    > -#define MAX_POSSIBLE_PASSTHROUGH_MSRS 13
    > +#define MAX_POSSIBLE_PASSTHROUGH_MSRS 14
    > struct {
    > DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
    > DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);

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