[PATCH v5 15/19] KVM:x86: Optimize CET supervisor SSP save/reload

Yang Weijiang
2023-08-03T07:38:14Z

Make PL{0,1,2}_SSP as write-intercepted to detect whether
guest is using these MSRs. Disable intercept to the MSRs
if they're written with non-zero values. KVM does save/
reload for the MSRs only if they're used by guest.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/vmx/vmx.c | 31 ++++++++++++++++++++++++++++---
arch/x86/kvm/x86.c | 8 ++++++--
3 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 69cbc9d9b277..c50b555234fb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -748,6 +748,7 @@ struct kvm_vcpu_arch {
bool tpr_access_reporting;
bool xsaves_enabled;
bool xfd_no_write_intercept;
+ bool cet_sss_active;
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 99bf63b2a779..96e22515ed13 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2152,6 +2152,18 @@ static u64 vmx_get_supported_debugctl(struct kvm_vcpu *vcpu, bool host_initiated
return debugctl;
}

+static void vmx_disable_write_intercept_sss_msr(struct kvm_vcpu *vcpu)
+{
+ if (guest_can_use(vcpu, X86_FEATURE_SHSTK)) {
+ vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
+ MSR_TYPE_RW, false);
+ vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
+ MSR_TYPE_RW, false);
+ vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
+ MSR_TYPE_RW, false);
+ }
+}
+
/*
* Writes msr value into the appropriate "register".
* Returns 0 on success, non-0 otherwise.
@@ -2420,6 +2432,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
else
vmx->pt_desc.guest.addr_a[index / 2] = data;
break;
+ case MSR_IA32_PL0_SSP ... MSR_IA32_PL2_SSP:
+ if (kvm_set_msr_common(vcpu, msr_info))
+ return 1;
+ if (data) {
+ vmx_disable_write_intercept_sss_msr(vcpu);
+ wrmsrl(msr_index, data);
+ }
+ break;
case MSR_IA32_S_CET:
case MSR_KVM_GUEST_SSP:
case MSR_IA32_INT_SSP_TAB:
@@ -7777,12 +7797,17 @@ static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
MSR_TYPE_RW, incpt);
vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
MSR_TYPE_RW, incpt);
+ /*
+ * Supervisor shadow stack MSRs are intercepted until
+ * they're written by guest, this is designed to
+ * optimize the save/restore overhead.
+ */
vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
- MSR_TYPE_RW, incpt);
+ MSR_TYPE_R, incpt);
vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
- MSR_TYPE_RW, incpt);
+ MSR_TYPE_R, incpt);
vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
- MSR_TYPE_RW, incpt);
+ MSR_TYPE_R, incpt);
vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
MSR_TYPE_RW, incpt);
vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 01b4f10fa8ab..fa3e7f7c639f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4060,6 +4060,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
msr == MSR_IA32_PL2_SSP) {
vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP] = data;
+ if (!vcpu->arch.cet_sss_active && data)
+ vcpu->arch.cet_sss_active = true;
} else if (msr == MSR_IA32_PL3_SSP) {
kvm_set_xsave_msr(msr_info);
}
@@ -11241,7 +11243,8 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)

void save_cet_supervisor_ssp(struct kvm_vcpu *vcpu)
{
- if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK))) {
+ if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
+ vcpu->arch.cet_sss_active)) {
rdmsrl(MSR_IA32_PL0_SSP, vcpu->arch.cet_s_ssp[0]);
rdmsrl(MSR_IA32_PL1_SSP, vcpu->arch.cet_s_ssp[1]);
rdmsrl(MSR_IA32_PL2_SSP, vcpu->arch.cet_s_ssp[2]);
@@ -11256,7 +11259,8 @@ EXPORT_SYMBOL_GPL(save_cet_supervisor_ssp);

void reload_cet_supervisor_ssp(struct kvm_vcpu *vcpu)
{
- if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK))) {
+ if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
+ vcpu->arch.cet_sss_active)) {
wrmsrl(MSR_IA32_PL0_SSP, vcpu->arch.cet_s_ssp[0]);
wrmsrl(MSR_IA32_PL1_SSP, vcpu->arch.cet_s_ssp[1]);
wrmsrl(MSR_IA32_PL2_SSP, vcpu->arch.cet_s_ssp[2]);
--
2.27.0