Commit 844d69c2 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace 



Use KVM's "user return MSRs" framework to defer restoring the host's
MSR_TSC_AUX until the CPU returns to userspace.  Add/improve comments to
clarify why MSR_TSC_AUX is intercepted on both RDMSR and WRMSR, and why
it's safe for KVM to keep the guest's value loaded even if KVM is
scheduled out.

Cc: Reiji Watanabe <reijiw@google.com>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20210423223404.3860547-5-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent dbd61273

arch/x86/kvm/svm/svm.c

+24 −29
Original line number Diff line number Diff line
@@ -213,6 +213,15 @@ struct kvm_ldttss_desc { 


DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);



/*

 * Only MSR_TSC_AUX is switched via the user return hook.  EFER is switched via

 * the VMCB, and the SYSCALL/SYSENTER MSRs are handled by VMLOAD/VMSAVE.

 *

 * RDTSCP and RDPID are not used in the kernel, specifically to allow KVM to

 * defer the restoration of TSC_AUX until the CPU returns to userspace.

 */

#define TSC_AUX_URET_SLOT	0



static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};



#define NUM_MSR_MAPS ARRAY_SIZE(msrpm_ranges)
@@ -958,6 +967,9 @@ static __init int svm_hardware_setup(void) 
		kvm_tsc_scaling_ratio_frac_bits = 32;

	}



	if (boot_cpu_has(X86_FEATURE_RDTSCP))

		kvm_define_user_return_msr(TSC_AUX_URET_SLOT, MSR_TSC_AUX);



	/* Check for pause filtering support */

	if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {

		pause_filter_count = 0;
@@ -1423,19 +1435,10 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu) 
{

	struct vcpu_svm *svm = to_svm(vcpu);

	struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);

	unsigned int i;



	if (svm->guest_state_loaded)

		return;



	/*

	 * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save

	 * area (non-sev-es). Save ones that aren't so we can restore them

	 * individually later.

	 */

	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)

		rdmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);



	/*

	 * Save additional host state that will be restored on VMEXIT (sev-es)

	 * or subsequent vmload of host save area.
@@ -1454,29 +1457,15 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu) 
		}

	}



	/* This assumes that the kernel never uses MSR_TSC_AUX */

	if (static_cpu_has(X86_FEATURE_RDTSCP))

		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);

		kvm_set_user_return_msr(TSC_AUX_URET_SLOT, svm->tsc_aux, -1ull);



	svm->guest_state_loaded = true;

}



static void svm_prepare_host_switch(struct kvm_vcpu *vcpu)

{

	struct vcpu_svm *svm = to_svm(vcpu);

	unsigned int i;



	if (!svm->guest_state_loaded)

		return;



	/*

	 * Certain MSRs are restored on VMEXIT (sev-es), or vmload of host save

	 * area (non-sev-es). Restore the ones that weren't.

	 */

	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)

		wrmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);



	svm->guest_state_loaded = false;

	to_svm(vcpu)->guest_state_loaded = false;

}



static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
@@ -2790,6 +2779,7 @@ static int svm_set_vm_cr(struct kvm_vcpu *vcpu, u64 data) 
static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)

{

	struct vcpu_svm *svm = to_svm(vcpu);

	int r;



	u32 ecx = msr->index;

	u64 data = msr->data;
@@ -2910,11 +2900,16 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) 
		data = (u32)data;



		/*

		 * This is rare, so we update the MSR here instead of using

		 * direct_access_msrs.  Doing that would require a rdmsr in

		 * svm_vcpu_put.

		 * TSC_AUX is usually changed only during boot and never read

		 * directly.  Intercept TSC_AUX instead of exposing it to the

		 * guest via direct_access_msrs, and switch it via user return.

		 */

		wrmsrl(MSR_TSC_AUX, data);

		preempt_disable();

		r = kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);

		preempt_enable();

		if (r)

			return 1;



		svm->tsc_aux = data;

		break;

	case MSR_IA32_DEBUGCTLMSR:

arch/x86/kvm/svm/svm.h

+0 −7
Original line number Diff line number Diff line
@@ -23,11 +23,6 @@ 


#define __sme_page_pa(x) __sme_set(page_to_pfn(x) << PAGE_SHIFT)



static const u32 host_save_user_msrs[] = {

	MSR_TSC_AUX,

};

#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)



#define	IOPM_SIZE PAGE_SIZE * 3

#define	MSRPM_SIZE PAGE_SIZE * 2
@@ -129,8 +124,6 @@ struct vcpu_svm { 


	u64 next_rip;



	u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];



	u64 spec_ctrl;

	/*

	 * Contains guest-controlled bits of VIRT_SPEC_CTRL, which will be