Re: [PATCH v2 1/3] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 5/24/2023 2:16 PM, Chao Gao wrote:
to avoid computing the supported value at runtime every time.

Toggle the ARCH_CAP_SKIP_VMENTRY_L1DFLUSH bit when l1tf_vmx_mitigation
is modified to achieve the same result as runtime computing.

It's not the same result.

In kvm_get_arch_capabilities(), host's value is honored. I.e., when host supports ARCH_CAP_SKIP_VMENTRY_L1DFLUSH, l1tf_vmx_mitigation doesn't make any difference to the result.

Opportunistically, add a comment to document the problem of allowing
changing the supported value of ARCH_CAPABILITIES and the reason why
we don't fix it.

No functional change intended.

Link: https://lore.kernel.org/all/ZGZhW%2Fx5OWPmx1qD@xxxxxxxxxx/
Link: https://lore.kernel.org/all/ZGeU9sYTPxqNGSqI@xxxxxxxxxx/
Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
  arch/x86/kvm/vmx/vmx.c | 25 +++++++++++++++++++++++--
  arch/x86/kvm/x86.c     |  7 ++++---
  arch/x86/kvm/x86.h     |  1 +
  3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 44fb619803b8..8274ef5e89e5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -309,10 +309,31 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
l1tf_vmx_mitigation = l1tf; - if (l1tf != VMENTER_L1D_FLUSH_NEVER)
+	/*
+	 * Update static keys and supported arch capabilities according to
+	 * the new mitigation state.
+	 *
+	 * ARCH_CAP_SKIP_VMENTRY_L1DFLUSH is toggled because if we do cache
+	 * flushes for L1 guests on (nested) vmlaunch/vmresume to L2, L1
+	 * guests can skip the flush and if we don't, then L1 guests need
+	 * to do a flush.
+	 *
+	 * Toggling ARCH_CAP_SKIP_VMENTRY_L1DFLUSH may present inconsistent
+	 * model to the guest, e.g., if userspace isn't careful, a VM can
+	 * have vCPUs with different values for ARCH_CAPABILITIES. But
+	 * there is almost no chance to fix the issue. Because, to present
+	 * a consistent model, KVM essentially needs to disallow changing
+	 * the module param after VMs/vCPUs have been created, but that
+	 * would prevent userspace from toggling the param while VMs are
+	 * running, e.g., in response to a new vulnerability.
+	 */
+	if (l1tf != VMENTER_L1D_FLUSH_NEVER) {
  		static_branch_enable(&vmx_l1d_should_flush);
-	else
+		kvm_caps.supported_arch_cap |= ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
+	} else {
  		static_branch_disable(&vmx_l1d_should_flush);
+		kvm_caps.supported_arch_cap &= ~ARCH_CAP_SKIP_VMENTRY_L1DFLUSH;
+	}
if (l1tf == VMENTER_L1D_FLUSH_COND)
  		static_branch_enable(&vmx_l1d_flush_cond);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c0778ca39650..2408b5f554b7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1672,7 +1672,7 @@ static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
  {
  	switch (msr->index) {
  	case MSR_IA32_ARCH_CAPABILITIES:
-		msr->data = kvm_get_arch_capabilities();
+		msr->data = kvm_caps.supported_arch_cap;
  		break;
  	case MSR_IA32_PERF_CAPABILITIES:
  		msr->data = kvm_caps.supported_perf_cap;
@@ -7156,7 +7156,7 @@ static void kvm_probe_msr_to_save(u32 msr_index)
  			return;
  		break;
  	case MSR_IA32_TSX_CTRL:
-		if (!(kvm_get_arch_capabilities() & ARCH_CAP_TSX_CTRL_MSR))
+		if (!(kvm_caps.supported_arch_cap & ARCH_CAP_TSX_CTRL_MSR))
  			return;
  		break;
  	default:
@@ -9532,6 +9532,7 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
  		kvm_caps.max_guest_tsc_khz = max;
  	}
  	kvm_caps.default_tsc_scaling_ratio = 1ULL << kvm_caps.tsc_scaling_ratio_frac_bits;
+	kvm_caps.supported_arch_cap = kvm_get_arch_capabilities();
  	kvm_init_msr_lists();
  	return 0;
@@ -11895,7 +11896,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
  	if (r)
  		goto free_guest_fpu;
- vcpu->arch.arch_capabilities = kvm_get_arch_capabilities();
+	vcpu->arch.arch_capabilities = kvm_caps.supported_arch_cap;
  	vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
  	kvm_xen_init_vcpu(vcpu);
  	kvm_vcpu_mtrr_init(vcpu);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index c544602d07a3..d3e524bcc169 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -29,6 +29,7 @@ struct kvm_caps {
  	u64 supported_xcr0;
  	u64 supported_xss;
  	u64 supported_perf_cap;
+	u64 supported_arch_cap;
  };
void kvm_spurious_fault(void);




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux