Re: [PATCH v5 11/19] KVM:VMX: Emulate read and write to CET MSRs

Chao Gao
2023-08-04T05:17:51Z

On Thu, Aug 03, 2023 at 12:27:24AM -0400, Yang Weijiang wrote:
>Add emulation interface for CET MSR read and write.
>The emulation code is split into common part and vendor specific
>part, the former resides in x86.c to benefic different x86 CPU
>vendors, the latter for VMX is implemented in this patch.
>
>Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>---
> arch/x86/kvm/vmx/vmx.c | 27 +++++++++++
> arch/x86/kvm/x86.c | 104 +++++++++++++++++++++++++++++++++++++----
> arch/x86/kvm/x86.h | 18 +++++++
> 3 files changed, 141 insertions(+), 8 deletions(-)
>
>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 6aa76124e81e..ccf750e79608 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -2095,6 +2095,18 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> else
> msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
> break;
>+ case MSR_IA32_S_CET:
>+ case MSR_KVM_GUEST_SSP:
>+ case MSR_IA32_INT_SSP_TAB:
>+ if (kvm_get_msr_common(vcpu, msr_info))
>+ return 1;
>+ if (msr_info->index == MSR_KVM_GUEST_SSP)
>+ msr_info->data = vmcs_readl(GUEST_SSP);
>+ else if (msr_info->index == MSR_IA32_S_CET)
>+ msr_info->data = vmcs_readl(GUEST_S_CET);
>+ else if (msr_info->index == MSR_IA32_INT_SSP_TAB)
>+ msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);

This if-else-if suggests that they are focibly grouped together to just
share the call of kvm_get_msr_common(). For readability, I think it is better
to handle them separately.

e.g.,
case MSR_IA32_S_CET:
if (kvm_get_msr_common(vcpu, msr_info))
return 1;
msr_info->data = vmcs_readl(GUEST_S_CET);
break;

case MSR_KVM_GUEST_SSP:
if (kvm_get_msr_common(vcpu, msr_info))
return 1;
msr_info->data = vmcs_readl(GUEST_SSP);
break;

...

>+ break;
> case MSR_IA32_DEBUGCTLMSR:
> msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> break;
>@@ -2404,6 +2416,18 @@ 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_S_CET:
>+ case MSR_KVM_GUEST_SSP:
>+ case MSR_IA32_INT_SSP_TAB:
>+ if (kvm_set_msr_common(vcpu, msr_info))
>+ return 1;
>+ if (msr_index == MSR_KVM_GUEST_SSP)
>+ vmcs_writel(GUEST_SSP, data);
>+ else if (msr_index == MSR_IA32_S_CET)
>+ vmcs_writel(GUEST_S_CET, data);
>+ else if (msr_index == MSR_IA32_INT_SSP_TAB)
>+ vmcs_writel(GUEST_INTR_SSP_TABLE, data);

ditto

>+ break;
> case MSR_IA32_PERF_CAPABILITIES:
> if (data && !vcpu_to_pmu(vcpu)->version)
> return 1;
>@@ -4864,6 +4888,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> vmcs_write64(GUEST_BNDCFGS, 0);
>
> vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0); /* 22.2.1 */
>+ vmcs_writel(GUEST_SSP, 0);
>+ vmcs_writel(GUEST_S_CET, 0);
>+ vmcs_writel(GUEST_INTR_SSP_TABLE, 0);

where are MSR_IA32_PL3_SSP and MSR_IA32_U_CET reset?

I thought that guest FPU would be reset in kvm_vcpu_reset(). But it turns out
only MPX states are reset in KVM while other FPU states are unchanged. This
is aligned with "Table 10.1 IA-32 and Intel® 64 Processor States Following
Power-up, Reset, or INIT"

Could you double confirm the hardware beahavior that CET states are reset to 0
on INIT? If CET states are reset, we need to handle CET_IA32_PL3_SSP and
MSR_IA32_U_CET like MPX.

>
> kvm_make_request(KVM_REQ_APIC_PAGE_RELOAD, vcpu);
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 5b63441fd2d2..98f3ff6078e6 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -3627,6 +3627,39 @@ static bool kvm_is_msr_to_save(u32 msr_index)
> return false;
> }
>
>+static inline bool is_shadow_stack_msr(u32 msr)
>+{
>+ return msr == MSR_IA32_PL0_SSP ||
>+ msr == MSR_IA32_PL1_SSP ||
>+ msr == MSR_IA32_PL2_SSP ||
>+ msr == MSR_IA32_PL3_SSP ||
>+ msr == MSR_IA32_INT_SSP_TAB ||
>+ msr == MSR_KVM_GUEST_SSP;
>+}
>+
>+static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
>+ struct msr_data *msr)
>+{
>+ if (is_shadow_stack_msr(msr->index)) {
>+ if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
>+ return false;
>+
>+ if (msr->index == MSR_KVM_GUEST_SSP)
>+ return msr->host_initiated;
>+
>+ return msr->host_initiated ||
>+ guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>+ }
>+
>+ if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
>+ !kvm_cpu_cap_has(X86_FEATURE_IBT))
>+ return false;
>+
>+ return msr->host_initiated ||
>+ guest_cpuid_has(vcpu, X86_FEATURE_IBT) ||
>+ guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>+}
>+
> int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> {
> u32 msr = msr_info->index;
>@@ -3981,6 +4014,45 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> vcpu->arch.guest_fpu.xfd_err = data;
> break;
> #endif
>+#define CET_EXCLUSIVE_BITS (CET_SUPPRESS | CET_WAIT_ENDBR)
>+#define CET_CTRL_RESERVED_BITS GENMASK(9, 6)
>+#define CET_SHSTK_MASK_BITS GENMASK(1, 0)
>+#define CET_IBT_MASK_BITS (GENMASK_ULL(5, 2) | \
>+ GENMASK_ULL(63, 10))
>+#define CET_LEG_BITMAP_BASE(data) ((data) >> 12)
>+ case MSR_IA32_U_CET:
>+ case MSR_IA32_S_CET:
>+ if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>+ return 1;
>+ if (!!(data & CET_CTRL_RESERVED_BITS))
>+ return 1;
>+ if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
>+ (data & CET_SHSTK_MASK_BITS))
>+ return 1;
>+ if (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
>+ (data & CET_IBT_MASK_BITS))
>+ return 1;
>+ if (!IS_ALIGNED(CET_LEG_BITMAP_BASE(data), 4) ||
>+ (data & CET_EXCLUSIVE_BITS) == CET_EXCLUSIVE_BITS)
>+ return 1;
>+ if (msr == MSR_IA32_U_CET)

can you add a comment before this if() statement like?
/* MSR_IA32_S_CET is handled by vendor code */

>+ case MSR_KVM_GUEST_SSP:
>+ case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>+ if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>+ return 1;
>+ if (is_noncanonical_address(data, vcpu))
>+ return 1;
>+ if (!IS_ALIGNED(data, 4))
>+ return 1;
>+ 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;
>+ } else if (msr == MSR_IA32_PL3_SSP) {
>+ kvm_set_xsave_msr(msr_info);
>+ }

brackets are not needed.

also add a comment for MSR_KVM_GUEST_SSP.

>+ break;
> default:
> if (kvm_pmu_is_valid_msr(vcpu, msr))
> return kvm_pmu_set_msr(vcpu, msr_info);
>@@ -4051,7 +4123,9 @@ static int get_msr_mce(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
>
> int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> {
>- switch (msr_info->index) {
>+ u32 msr = msr_info->index;
>+
>+ switch (msr) {
> case MSR_IA32_PLATFORM_ID:
> case MSR_IA32_EBL_CR_POWERON:
> case MSR_IA32_LASTBRANCHFROMIP:
>@@ -4086,7 +4160,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_K7_PERFCTR0 ... MSR_K7_PERFCTR3:
> case MSR_P6_PERFCTR0 ... MSR_P6_PERFCTR1:
> case MSR_P6_EVNTSEL0 ... MSR_P6_EVNTSEL1:
>- if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>+ if (kvm_pmu_is_valid_msr(vcpu, msr))
> return kvm_pmu_get_msr(vcpu, msr_info);
> msr_info->data = 0;
> break;
>@@ -4137,7 +4211,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_MTRRcap:
> case MTRRphysBase_MSR(0) ... MSR_MTRRfix4K_F8000:
> case MSR_MTRRdefType:
>- return kvm_mtrr_get_msr(vcpu, msr_info->index, &msr_info->data);
>+ return kvm_mtrr_get_msr(vcpu, msr, &msr_info->data);
> case 0xcd: /* fsb frequency */
> msr_info->data = 3;
> break;
>@@ -4159,7 +4233,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> msr_info->data = kvm_get_apic_base(vcpu);
> break;
> case APIC_BASE_MSR ... APIC_BASE_MSR + 0xff:
>- return kvm_x2apic_msr_read(vcpu, msr_info->index, &msr_info->data);
>+ return kvm_x2apic_msr_read(vcpu, msr, &msr_info->data);
> case MSR_IA32_TSC_DEADLINE:
> msr_info->data = kvm_get_lapic_tscdeadline_msr(vcpu);
> break;
>@@ -4253,7 +4327,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_MCG_STATUS:
> case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
> case MSR_IA32_MC0_CTL2 ... MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) - 1:
>- return get_msr_mce(vcpu, msr_info->index, &msr_info->data,
>+ return get_msr_mce(vcpu, msr, &msr_info->data,
> msr_info->host_initiated);
> case MSR_IA32_XSS:
> if (!msr_info->host_initiated &&
>@@ -4284,7 +4358,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case HV_X64_MSR_TSC_EMULATION_STATUS:
> case HV_X64_MSR_TSC_INVARIANT_CONTROL:
> return kvm_hv_get_msr_common(vcpu,
>- msr_info->index, &msr_info->data,
>+ msr, &msr_info->data,
> msr_info->host_initiated);
> case MSR_IA32_BBL_CR_CTL3:
> /* This legacy MSR exists but isn't fully documented in current
>@@ -4337,8 +4411,22 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> msr_info->data = vcpu->arch.guest_fpu.xfd_err;
> break;
> #endif
>+ case MSR_IA32_U_CET:
>+ case MSR_IA32_S_CET:
>+ case MSR_KVM_GUEST_SSP:
>+ case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>+ if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>+ return 1;
>+ if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
>+ msr == MSR_IA32_PL2_SSP) {
>+ msr_info->data =
>+ vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP];
>+ } else if (msr == MSR_IA32_U_CET || msr == MSR_IA32_PL3_SSP) {
>+ kvm_get_xsave_msr(msr_info);
>+ }

Again, for readability and clarity, how about:

case MSR_IA32_U_CET:
case MSR_IA32_PL3_SSP:
if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
return 1;
kvm_get_xsave_msr(msr_info);
break;
case MSR_IA32_PL0_SSP ... MSR_IA32_PL2_SSP:
if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
return 1;
msr_info->data = vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP];
break;
case MSR_IA32_S_CET:
case MSR_KVM_GUEST_SSP:
if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
return 1;
/* Further handling in vendor code */
break;

>+ break;
> default:
>- if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>+ if (kvm_pmu_is_valid_msr(vcpu, msr))
> return kvm_pmu_get_msr(vcpu, msr_info);
>
> /*
>@@ -4346,7 +4434,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> * to-be-saved, even if an MSR isn't fully supported.
> */
> if (msr_info->host_initiated &&
>- kvm_is_msr_to_save(msr_info->index)) {
>+ kvm_is_msr_to_save(msr)) {
> msr_info->data = 0;
> break;
> }
>diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>index c69fc027f5ec..3b79d6db2f83 100644
>--- a/arch/x86/kvm/x86.h
>+++ b/arch/x86/kvm/x86.h
>@@ -552,4 +552,22 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> unsigned int port, void *data, unsigned int count,
> int in);
>
>+/*
>+ * Guest xstate MSRs have been loaded in __msr_io(), disable preemption before
>+ * access the MSRs to avoid MSR content corruption.
>+ */

I think it is better to describe what the function does prior to jumping into
details like where guest FPU is loaded.

/*
* Lock and/or reload guest FPU and access xstate MSRs. For accesses initiated
* by host, guest FPU is loaded in __msr_io(). For accesses initiated by guest,
* guest FPU should have been loaded already.
*/
>+static inline void kvm_get_xsave_msr(struct msr_data *msr_info)
>+{
>+ kvm_fpu_get();
>+ rdmsrl(msr_info->index, msr_info->data);
>+ kvm_fpu_put();
>+}
>+
>+static inline void kvm_set_xsave_msr(struct msr_data *msr_info)
>+{
>+ kvm_fpu_get();
>+ wrmsrl(msr_info->index, msr_info->data);
>+ kvm_fpu_put();
>+}

Can you rename functions to kvm_get/set_xstate_msr() to align with the comment
and patch 6? And if there is no user outside x86.c, you can just put these two
functions right after the is_xstate_msr() added in patch 6.

>+
> #endif
>--
>2.27.0
>