Re: [PATCH v3 3/8] KVM:CPUID: Add CPUID support for Guest CET

[Sean Christopherson]

On Mon, Feb 25, 2019 at 09:27:11PM +0800, Yang Weijiang wrote:
> Guest CET SHSTK and IBT capability are reported via
> CPUID.(EAX=7, ECX=0):ECX[bit 7] and EDX[bit 20] respectively.
> Guest user mode and supervisor mode xsaves component size
> is reported via CPUID.(EAX=0xD, ECX=1):ECX[bit 11] and ECX[bit 12]
> respectively.
> 
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 60 +++++++++++++++++++++++++++++++++-----------
>  arch/x86/kvm/x86.h   |  4 +++
>  2 files changed, 50 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index cb1aece25b17..5e05756cc6db 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -65,6 +65,16 @@ u64 kvm_supported_xcr0(void)
>  	return xcr0;
>  }
>  
> +u64 kvm_supported_xss(void)
> +{
> +	u64 xss;
> +
> +	rdmsrl(MSR_IA32_XSS, xss);

Honest question as I haven't thought through the flows: do we actually
need to restrict XSS based on what's enabled in the host?  This
conflicts with your other statements that CET features can be enabled
independent of host support.

And if we do incorporate the host status, the value should be read once
and cached unless we're expecting the host value to change dynamically,
e.g. see host_efer.

> +	xss &= KVM_SUPPORTED_XSS;
> +	return xss;
> +}
> +EXPORT_SYMBOL(kvm_supported_xss);
> +
>  #define F(x) bit(X86_FEATURE_##x)
>  
>  /* For scattered features from cpufeatures.h; we currently expose none */
> @@ -323,6 +333,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  				 u32 index, int *nent, int maxnent)
>  {
>  	int r;
> +	u32 eax, ebx, ecx, edx;
>  	unsigned f_nx = is_efer_nx() ? F(NX) : 0;
>  #ifdef CONFIG_X86_64
>  	unsigned f_gbpages = (kvm_x86_ops->get_lpage_level() == PT_PDPE_LEVEL)
> @@ -503,6 +514,20 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			 * if the host doesn't support it.
>  			 */
>  			entry->edx |= F(ARCH_CAPABILITIES);
> +
> +			/*
> +			 * Guest OS CET enabling is designed independent to
> +			 * host enabling, it only has dependency on Host HW
> +			 * capability, if it has, report CET support to
> +			 * Guest.
> +			 */
> +			cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
> +			if (ecx & F(SHSTK))
> +				entry->ecx |= F(SHSTK);
> +
> +			if (edx & F(IBT))
> +				entry->edx |= F(IBT);
> +
>  		} else {
>  			entry->ebx = 0;
>  			entry->ecx = 0;
> @@ -564,14 +589,17 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  	}
>  	case 0xd: {
>  		int idx, i;
> -		u64 supported = kvm_supported_xcr0();
> +		u64 u_supported = kvm_supported_xcr0();
> +		u64 s_supported = kvm_supported_xss();
> +		u64 supported;
> +		int compacted;
>  
> -		entry->eax &= supported;
> -		entry->ebx = xstate_required_size(supported, false);
> +		entry->eax &= u_supported;
> +		entry->ebx = xstate_required_size(u_supported, false);
>  		entry->ecx = entry->ebx;
> -		entry->edx &= supported >> 32;
> +		entry->edx &= u_supported >> 32;
>  		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> -		if (!supported)
> +		if (!u_supported && !s_supported)
>  			break;
>  
>  		for (idx = 1, i = 1; idx < 64; ++idx) {
> @@ -583,19 +611,23 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			if (idx == 1) {
>  				entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
>  				cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
> -				entry[i].ebx = 0;
> -				if (entry[i].eax & (F(XSAVES)|F(XSAVEC)))
> -					entry[i].ebx =
> -						xstate_required_size(supported,
> -								     true);
> +				supported = u_supported | s_supported;
> +				compacted = entry[i].eax &
> +					(F(XSAVES) | F(XSAVEC));
> +				entry[i].ebx = xstate_required_size(supported,
> +								    compacted);
> +				entry[i].ecx &= s_supported;
> +				entry[i].edx = 0;
>  			} else {
> +				supported = (entry[i].ecx & 1) ? s_supported :
> +								 u_supported;
>  				if (entry[i].eax == 0 || !(supported & mask))
>  					continue;
> -				if (WARN_ON_ONCE(entry[i].ecx & 1))
> -					continue;
> +				entry[i].ecx &= 1;
> +				entry[i].edx = 0;
> +				if (entry[i].ecx)
> +					entry[i].ebx = 0;
>  			}
> -			entry[i].ecx = 0;
> -			entry[i].edx = 0;
>  			entry[i].flags |=
>  			       KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>  			++*nent;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 224cd0a47568..c61da41c3c5c 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -283,6 +283,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, unsigned long cr2,
>  				| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
>  				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
>  				| XFEATURE_MASK_PKRU)
> +
> +#define KVM_SUPPORTED_XSS	(XFEATURE_MASK_SHSTK_USER \
> +				| XFEATURE_MASK_SHSTK_KERNEL)

I don't think these definitions are correct, the spec I have lists them
as CET_U and CET_S, i.e. they aren't specific to SHSTK.  Did these names
get inherited from the kernel enabling patches?

> +
>  extern u64 host_xcr0;
>  
>  extern u64 kvm_supported_xcr0(void);
> -- 
> 2.17.1
>