Re: [PATCH v6 23/25] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

From: Maxim Levitsky
Date: Tue Oct 31 2023 - 13:57:58 EST

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On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> Expose CET features to guest if KVM/host can support them, clear CPUID
> feature bits if KVM/host cannot support.
>
> Set CPUID feature bits so that CET features are available in guest CPUID.
> Add CR4.CET bit support in order to allow guest set CET master control
> bit.
>
> Disable KVM CET feature if unrestricted_guest is unsupported/disabled as
> KVM does not support emulating CET.
> Don't expose CET feature if either of {U,S}_CET xstate bits is cleared
> in host XSS or if XSAVES isn't supported.
>
> The CET load-bits in VM_ENTRY/VM_EXIT control fields should be set to make
> guest CET xstates isolated from host's. And all platforms that support CET
> enumerate VMX_BASIC[bit56] as 1, clear CET feature bits if the bit doesn't
> read 1.
>
> Regarding the CET MSR contents after Reset/INIT, SDM doesn't mention the
> default values, neither can I get the answer internally so far, will fill
> the gap once it's clear.
>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 3 ++-
> arch/x86/include/asm/msr-index.h | 1 +
> arch/x86/kvm/cpuid.c | 12 ++++++++++--
> arch/x86/kvm/vmx/capabilities.h | 6 ++++++
> arch/x86/kvm/vmx/vmx.c | 23 ++++++++++++++++++++++-
> arch/x86/kvm/vmx/vmx.h | 6 ++++--
> arch/x86/kvm/x86.c | 12 +++++++++++-
> arch/x86/kvm/x86.h | 3 +++
> 8 files changed, 59 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index d77b030e996c..db0010fa3363 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -125,7 +125,8 @@
> | X86_CR4_PGE | X86_CR4_PCE | X86_CR4_OSFXSR | X86_CR4_PCIDE \
> | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
> | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
> - | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP))
> + | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> + | X86_CR4_CET))
>
> #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 1d111350197f..1f8dc04da468 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -1091,6 +1091,7 @@
> #define VMX_BASIC_MEM_TYPE_MASK 0x003c000000000000LLU
> #define VMX_BASIC_MEM_TYPE_WB 6LLU
> #define VMX_BASIC_INOUT 0x0040000000000000LLU
> +#define VMX_BASIC_NO_HW_ERROR_CODE_CC 0x0100000000000000LLU
>
> /* Resctrl MSRs: */
> /* - Intel: */
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 4e7a820cba62..d787a506746a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -654,7 +654,7 @@ void kvm_set_cpu_caps(void)
> F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
> F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
> F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
> - F(SGX_LC) | F(BUS_LOCK_DETECT)
> + F(SGX_LC) | F(BUS_LOCK_DETECT) | F(SHSTK)
> );
> /* Set LA57 based on hardware capability. */
> if (cpuid_ecx(7) & F(LA57))
> @@ -672,7 +672,8 @@ void kvm_set_cpu_caps(void)
> F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
> F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
> F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
> - F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D)
> + F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D) |
> + F(IBT)
> );
>
> /* TSC_ADJUST and ARCH_CAPABILITIES are emulated in software. */
> @@ -685,6 +686,13 @@ void kvm_set_cpu_caps(void)
> kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
> kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> + /*
> + * The feature bit in boot_cpu_data.x86_capability could have been
> + * cleared due to ibt=off cmdline option, then add it back if CPU
> + * supports IBT.
> + */
> + if (cpuid_edx(7) & F(IBT))
> + kvm_cpu_cap_set(X86_FEATURE_IBT);

The usual policy is that when the host doesn't support a feature, then the guest
should not support it either. On the other hand, for this particular feature,
it is probably safe to use it. Just a point for a discussion.

>
> kvm_cpu_cap_mask(CPUID_7_1_EAX,
> F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) |
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index ee8938818c8a..e12bc233d88b 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -79,6 +79,12 @@ static inline bool cpu_has_vmx_basic_inout(void)
> return (((u64)vmcs_config.basic_cap << 32) & VMX_BASIC_INOUT);
> }
>
> +static inline bool cpu_has_vmx_basic_no_hw_errcode(void)
> +{
> + return ((u64)vmcs_config.basic_cap << 32) &
> + VMX_BASIC_NO_HW_ERROR_CODE_CC;
> +}

I see, this is because #CP does have an error code but when bit 56 of IA32_VMX_BASIC,
is clear then error code must be present iff exception is within a hardcoded list of exceptions,
and #CP is not on this list because back then the #CP didn't exist, and all new CPUs do
have this bit 56 set.

But I am not 100% sure that this check is worth it, I don't mind having it though,
but please add a comment explaining why the bit 56 is needed for CET.

> +
> static inline bool cpu_has_virtual_nmis(void)
> {
> return vmcs_config.pin_based_exec_ctrl & PIN_BASED_VIRTUAL_NMIS &&
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9ccc2c552f55..f0dea8ecd0c6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2614,6 +2614,7 @@ static int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> { VM_ENTRY_LOAD_IA32_EFER, VM_EXIT_LOAD_IA32_EFER },
> { VM_ENTRY_LOAD_BNDCFGS, VM_EXIT_CLEAR_BNDCFGS },
> { VM_ENTRY_LOAD_IA32_RTIT_CTL, VM_EXIT_CLEAR_IA32_RTIT_CTL },
> + { VM_ENTRY_LOAD_CET_STATE, VM_EXIT_LOAD_CET_STATE },
> };
>
> memset(vmcs_conf, 0, sizeof(*vmcs_conf));
> @@ -4934,6 +4935,9 @@ 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 */

I guess that the below 3 writes should only be done if CET is supported,
this is what the kernel test robot is complaining about.

> + vmcs_writel(GUEST_SSP, 0);
> + vmcs_writel(GUEST_S_CET, 0);
> + vmcs_writel(GUEST_INTR_SSP_TABLE, 0);
>
> kvm_make_request(KVM_REQ_APIC_PAGE_RELOAD, vcpu);
>
> @@ -6354,6 +6358,12 @@ void dump_vmcs(struct kvm_vcpu *vcpu)
> if (vmcs_read32(VM_EXIT_MSR_STORE_COUNT) > 0)
> vmx_dump_msrs("guest autostore", &vmx->msr_autostore.guest);
>
> + if (vmentry_ctl & VM_ENTRY_LOAD_CET_STATE) {
> + pr_err("S_CET = 0x%016lx\n", vmcs_readl(GUEST_S_CET));
> + pr_err("SSP = 0x%016lx\n", vmcs_readl(GUEST_SSP));
> + pr_err("INTR SSP TABLE = 0x%016lx\n",
> + vmcs_readl(GUEST_INTR_SSP_TABLE));
> + }
> pr_err("*** Host State ***\n");
> pr_err("RIP = 0x%016lx RSP = 0x%016lx\n",
> vmcs_readl(HOST_RIP), vmcs_readl(HOST_RSP));
> @@ -6431,6 +6441,12 @@ void dump_vmcs(struct kvm_vcpu *vcpu)
> if (secondary_exec_control & SECONDARY_EXEC_ENABLE_VPID)
> pr_err("Virtual processor ID = 0x%04x\n",
> vmcs_read16(VIRTUAL_PROCESSOR_ID));
> + if (vmexit_ctl & VM_EXIT_LOAD_CET_STATE) {
> + pr_err("S_CET = 0x%016lx\n", vmcs_readl(HOST_S_CET));
> + pr_err("SSP = 0x%016lx\n", vmcs_readl(HOST_SSP));
> + pr_err("INTR SSP TABLE = 0x%016lx\n",
> + vmcs_readl(HOST_INTR_SSP_TABLE));
> + }
> }
>
> /*
> @@ -7967,7 +7983,6 @@ static __init void vmx_set_cpu_caps(void)
> kvm_cpu_cap_set(X86_FEATURE_UMIP);
>
> /* CPUID 0xD.1 */
> - kvm_caps.supported_xss = 0;
> if (!cpu_has_vmx_xsaves())
> kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
>
> @@ -7979,6 +7994,12 @@ static __init void vmx_set_cpu_caps(void)
>
> if (cpu_has_vmx_waitpkg())
> kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
> +
> + if (!cpu_has_load_cet_ctrl() || !enable_unrestricted_guest ||
> + !cpu_has_vmx_basic_no_hw_errcode()) {
> + kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> + kvm_cpu_cap_clear(X86_FEATURE_IBT);

I think that here we also need to clear kvm_caps.supported_xss,
or even better, lets set the CET bits in kvm_caps.supported_xss only
once CET is fully enabled (both this check and check in __kvm_x86_vendor_init pass).

> + }
> }
>
> static void vmx_request_immediate_exit(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index c2130d2c8e24..fb72819fbb41 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -480,7 +480,8 @@ static inline u8 vmx_get_rvi(void)
> VM_ENTRY_LOAD_IA32_EFER | \
> VM_ENTRY_LOAD_BNDCFGS | \
> VM_ENTRY_PT_CONCEAL_PIP | \
> - VM_ENTRY_LOAD_IA32_RTIT_CTL)
> + VM_ENTRY_LOAD_IA32_RTIT_CTL | \
> + VM_ENTRY_LOAD_CET_STATE)
>
> #define __KVM_REQUIRED_VMX_VM_EXIT_CONTROLS \
> (VM_EXIT_SAVE_DEBUG_CONTROLS | \
> @@ -502,7 +503,8 @@ static inline u8 vmx_get_rvi(void)
> VM_EXIT_LOAD_IA32_EFER | \
> VM_EXIT_CLEAR_BNDCFGS | \
> VM_EXIT_PT_CONCEAL_PIP | \
> - VM_EXIT_CLEAR_IA32_RTIT_CTL)
> + VM_EXIT_CLEAR_IA32_RTIT_CTL | \
> + VM_EXIT_LOAD_CET_STATE)
>
> #define KVM_REQUIRED_VMX_PIN_BASED_VM_EXEC_CONTROL \
> (PIN_BASED_EXT_INTR_MASK | \
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 231d4a7b6f3d..b7d1ac6b8d75 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -231,7 +231,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
> | XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> | XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
>
> -#define KVM_SUPPORTED_XSS 0
> +#define KVM_SUPPORTED_XSS (XFEATURE_MASK_CET_USER | \
> + XFEATURE_MASK_CET_KERNEL)
>
> u64 __read_mostly host_efer;
> EXPORT_SYMBOL_GPL(host_efer);
> @@ -9699,6 +9700,15 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
> kvm_caps.supported_xss = 0;
>
> + if ((kvm_caps.supported_xss & (XFEATURE_MASK_CET_USER |
> + XFEATURE_MASK_CET_KERNEL)) !=
> + (XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)) {
> + kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> + kvm_cpu_cap_clear(X86_FEATURE_IBT);
> + kvm_caps.supported_xss &= ~XFEATURE_CET_USER;
> + kvm_caps.supported_xss &= ~XFEATURE_CET_KERNEL;
> + }
> +
> #define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
> cr4_reserved_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_);
> #undef __kvm_cpu_cap_has
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 0d5f673338dd..665a7f91d04f 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -530,6 +530,9 @@ bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type);
> __reserved_bits |= X86_CR4_VMXE; \
> if (!__cpu_has(__c, X86_FEATURE_PCID)) \
> __reserved_bits |= X86_CR4_PCIDE; \
> + if (!__cpu_has(__c, X86_FEATURE_SHSTK) && \
> + !__cpu_has(__c, X86_FEATURE_IBT)) \
> + __reserved_bits |= X86_CR4_CET; \
> __reserved_bits; \
> })
>

Best regards,
Maxim Levitsky

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