From: Kevin Tian kevin.tian@intel.com
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.
Always trap #NM once write interception is disabled for IA32_XFD. The #NM exception is rare if the guest doesn't use dynamic features. Otherwise, there is at most one exception per guest task given a dynamic feature.
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: Jing Liu jing2.liu@intel.com Signed-off-by: Yang Zhong yang.zhong@intel.com --- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/vmx/vmx.c | 23 ++++++++++++++++++----- arch/x86/kvm/vmx/vmx.h | 2 +- arch/x86/kvm/x86.c | 3 +++ 4 files changed, 23 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 555f4de47ef2..a372dfe6f407 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -640,6 +640,7 @@ struct kvm_vcpu_arch { u64 smi_count; bool tpr_access_reporting; bool xsaves_enabled; + bool xfd_no_write_intercept; 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 638665b3e241..21c4d7409433 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, @@ -766,10 +767,11 @@ void vmx_update_exception_bitmap(struct kvm_vcpu *vcpu) }
/* - * Trap #NM if guest xfd contains a non-zero value so guest XFD_ERR - * can be saved timely. + * Disabling xfd interception indicates that dynamically-enabled + * features might be used in the guest. Always trap #NM in this case + * for proper virtualization of guest xfd_err. */ - if (vcpu->arch.guest_fpu.fpstate->xfd) + if (vcpu->arch.xfd_no_write_intercept) eb |= (1u << NM_VECTOR);
vmcs_write32(EXCEPTION_BITMAP, eb); @@ -1971,9 +1973,20 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info) break; case MSR_IA32_XFD: ret = kvm_set_msr_common(vcpu, msr_info); - /* Update #NM interception according to guest xfd */ - if (!ret) + /* + * Always intercepting WRMSR could incur non-negligible + * overhead given xfd might be touched in guest context + * switch. Disable write interception upon the first write + * with a non-zero value (indicate potential guest usage + * on dynamic xfeatures). Also update exception bitmap + * to trap #NM for proper virtualization of guest xfd_err. + */ + if (!ret && data) { + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD, + MSR_TYPE_RW); + vcpu->arch.xfd_no_write_intercept = true; vmx_update_exception_bitmap(vcpu); + } break; #endif case MSR_IA32_SYSENTER_CS: 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 76e1941db223..4990d6de5359 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -10027,6 +10027,9 @@ 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_no_write_intercept) + 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.