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    Subject[PATCH v3 22/22] kvm: x86: Disable interception for IA32_XFD on demand
    Date
    Always intercepting IA32_XFD causes non-negligible overhead when this
    register is updated frequently in the guest.

    Disable r/w emulation after intercepting the first WRMSR(IA32_XFD)
    with a non-zero value.

    Disable WRMSR emulation implies that IA32_XFD becomes out-of-sync
    with the software states in fpstate and the per-cpu xfd cache. Call
    fpu_sync_guest_vmexit_xfd_state() to bring them back in-sync, before
    preemption is enabled.

    p.s. We have confirmed that SDM is being revised to say that
    when setting IA32_XFD[18] the AMX register state is not guaranteed
    to be preserved. This clarification avoids adding mess for a creative
    guest which sets IA32_XFD[18]=1 before saving active AMX state to
    its own storage.

    Signed-off-by: Kevin Tian <kevin.tian@intel.com>
    Signed-off-by: Yang Zhong <yang.zhong@intel.com>
    Signed-off-by: Jing Liu <jing2.liu@intel.com>
    ---
    arch/x86/include/asm/kvm_host.h | 1 +
    arch/x86/kvm/vmx/vmx.c | 4 ++++
    arch/x86/kvm/vmx/vmx.h | 2 +-
    arch/x86/kvm/x86.c | 3 +++
    4 files changed, 9 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    index f7a661f35d1a..7b81d324b32a 100644
    --- a/arch/x86/include/asm/kvm_host.h
    +++ b/arch/x86/include/asm/kvm_host.h
    @@ -641,6 +641,7 @@ struct kvm_vcpu_arch {
    bool tpr_access_reporting;
    bool xsaves_enabled;
    bool trap_nm;
    + bool xfd_out_of_sync;
    u64 ia32_xss;
    u64 microcode_version;
    u64 arch_capabilities;
    diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    index 8595a3e8fbd8..7aaf905e13b4 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,
    MSR_IA32_XFD_ERR,
    #endif
    MSR_IA32_SYSENTER_CS,
    @@ -1968,6 +1969,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
    case MSR_IA32_XFD:
    ret = kvm_set_msr_common(vcpu, msr_info);
    if (!ret && data) {
    + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD, MSR_TYPE_RW);
    + vcpu->arch.xfd_out_of_sync = true;
    +
    vcpu->arch.trap_nm = true;
    vmx_update_exception_bitmap(vcpu);
    }
    diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
    index bf9d3051cd6c..0a00242a91e7 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 14
    +#define MAX_POSSIBLE_PASSTHROUGH_MSRS 15
    struct {
    DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
    DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);
    diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    index 3b756ff13103..10a08aa2aa45 100644
    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -10024,6 +10024,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
    if (vcpu->arch.guest_fpu.xfd_err)
    wrmsrl(MSR_IA32_XFD_ERR, 0);

    + if (vcpu->arch.xfd_out_of_sync)
    + fpu_sync_guest_vmexit_xfd_state();
    +
    /*
    * Consume any pending interrupts, including the possible source of
    * VM-Exit on SVM and any ticks that occur between VM-Exit and now.
    --
    2.27.0
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