Commit b5274b1b authored by Kevin Tian's avatar Kevin Tian Committed by Paolo Bonzini
Browse files

kvm: x86: Disable interception for IA32_XFD on demand 



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. This
leads to two additional changes accordingly:

  - Call fpu_sync_guest_vmexit_xfd_state() after vm-exit to bring
    software states back in-sync with the MSR, before handle_exit_irqoff()
    is called.

  - 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: default avatarKevin Tian <kevin.tian@intel.com>
Signed-off-by: default avatarJing Liu <jing2.liu@intel.com>
Signed-off-by: default avatarYang Zhong <yang.zhong@intel.com>
Message-Id: <20220105123532.12586-22-yang.zhong@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 5429cead

arch/x86/include/asm/kvm_host.h

+1 −0
Original line number Diff line number Diff line
@@ -647,6 +647,7 @@ 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;

arch/x86/kvm/vmx/vmx.c

+19 −5
Original line number Diff line number Diff line
@@ -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,
@@ -764,10 +765,11 @@ 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 dynamic xfeatures

	 * might be used in the guest. Always trap #NM in this case

	 * to save guest xfd_err timely.

	 */

	if (vcpu->arch.guest_fpu.fpstate->xfd)

	if (vcpu->arch.xfd_no_write_intercept)

		eb |= (1u << NM_VECTOR);



	vmcs_write32(EXCEPTION_BITMAP, eb);
@@ -1978,9 +1980,21 @@ 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 changed frequently in

		 * guest context switch. Disable write interception

		 * upon the first write with a non-zero value (indicating

		 * potential 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:

arch/x86/kvm/vmx/vmx.h

+1 −1
Original line number Diff line number Diff line
@@ -349,7 +349,7 @@ 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);

arch/x86/kvm/x86.c

+8 −0
Original line number Diff line number Diff line
@@ -10083,6 +10083,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) 
	vcpu->mode = OUTSIDE_GUEST_MODE;

	smp_wmb();



	/*

	 * Sync xfd before calling handle_exit_irqoff() which may

	 * rely on the fact that guest_fpu::xfd is up-to-date (e.g.

	 * in #NM irqoff handler).

	 */

	if (vcpu->arch.xfd_no_write_intercept)

		fpu_sync_guest_vmexit_xfd_state();



	static_call(kvm_x86_handle_exit_irqoff)(vcpu);



	if (vcpu->arch.guest_fpu.xfd_err)