Re: [PATCH v1 2/3] x86/bugs: Add AMD's SPEC_CTRL MSR usage

[Tom Lendacky <thomas.lendacky@amd.com>]

On 6/1/2018 9:59 AM, Konrad Rzeszutek Wilk wrote:

Hi Konrad,

Thanks for doing this.  It was on my to-do list to get this
support out after everything settled down.

Just some questions/comments below.

> The AMD document outlining the SSBD handling
> 124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
> mentions that if CPUID 8000_0008.EBX[24] is set we should be using
> the SPEC_CTRL MSR (0x48) over the VIRT SPEC_CTRL MSR (0xC001_011f)
> for speculative store bypass disable.
> 
> This in effect means we should clear the X86_FEATURE_VIRT_SSBD
> flag so that we would prefer the SPEC_CTRL MSR.
> 
> See the document titled:
> 124441_AMD64_SpeculativeStoreBypassDisable_Whitepaper_final.pdf
> 
> A copy of this document is available at
>    https://bugzilla.kernel.org/show_bug.cgi?id=199889
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> 
> ---
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: KarimAllah Ahmed <karahmed@amazon.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> ---
>  arch/x86/include/asm/cpufeatures.h |  1 +
>  arch/x86/kernel/cpu/bugs.c         | 12 +++++++-----
>  arch/x86/kernel/cpu/common.c       |  6 ++++++
>  arch/x86/kvm/cpuid.c               | 10 ++++++++--
>  arch/x86/kvm/svm.c                 |  8 +++++---
>  5 files changed, 27 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index b6d7ce32927a..5701f5cecd31 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -282,6 +282,7 @@
>  #define X86_FEATURE_AMD_IBPB		(13*32+12) /* "" Indirect Branch Prediction Barrier */
>  #define X86_FEATURE_AMD_IBRS		(13*32+14) /* "" Indirect Branch Restricted Speculation */
>  #define X86_FEATURE_AMD_STIBP		(13*32+15) /* "" Single Thread Indirect Branch Predictors */
> +#define X86_FEATURE_AMD_SSBD		(13*32+24) /* "" Speculative Store Bypass Disable */
>  #define X86_FEATURE_VIRT_SSBD		(13*32+25) /* Virtualized Speculative Store Bypass Disable */
>  #define X86_FEATURE_AMD_SSB_NO		(13*32+26) /* "" Speculative Store Bypass is fixed in hardware. */
>  
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 7416fc206b4a..6bea81855cdd 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -529,18 +529,20 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
>  	if (mode == SPEC_STORE_BYPASS_DISABLE) {
>  		setup_force_cpu_cap(X86_FEATURE_SPEC_STORE_BYPASS_DISABLE);
>  		/*
> -		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD uses
> -		 * a completely different MSR and bit dependent on family.
> +		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD may
> +		 * use a completely different MSR and bit dependent on family.
>  		 */
>  		switch (boot_cpu_data.x86_vendor) {
>  		case X86_VENDOR_INTEL:
> +		case X86_VENDOR_AMD:
> +			if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
> +				x86_amd_ssb_disable();
> +				break;
> +			}
>  			x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
>  			x86_spec_ctrl_mask |= SPEC_CTRL_SSBD;
>  			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
>  			break;
> -		case X86_VENDOR_AMD:
> -			x86_amd_ssb_disable();
> -			break;
>  		}
>  	}
>  
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 494735cf63f5..d08a29bd0385 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -783,6 +783,12 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
>  		set_cpu_cap(c, X86_FEATURE_STIBP);
>  		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
>  	}
> +
> +	if (cpu_has(c, X86_FEATURE_AMD_SSBD)) {
> +		set_cpu_cap(c, X86_FEATURE_SSBD);
> +		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
> +		clear_cpu_cap(c, X86_FEATURE_VIRT_SSBD);
> +	}
>  }
>  
>  void get_cpu_cap(struct cpuinfo_x86 *c)
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 132f8a58692e..f4f30d0c25c4 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -379,7 +379,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  
>  	/* cpuid 0x80000008.ebx */
>  	const u32 kvm_cpuid_8000_0008_ebx_x86_features =
> -		F(AMD_IBPB) | F(AMD_IBRS) | F(VIRT_SSBD) | F(AMD_SSB_NO);
> +		F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
> +		F(AMD_SSB_NO);
>  
>  	/* cpuid 0xC0000001.edx */
>  	const u32 kvm_cpuid_C000_0001_edx_x86_features =
> @@ -664,7 +665,12 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			entry->ebx |= F(VIRT_SSBD);
>  		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
>  		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
> -		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD))
> +		/*
> +		 * The preference is to use SPEC CTRL MSR instead of the
> +		 * VIRT_SPEC MSR.
> +		 */
> +		if (boot_cpu_has(X86_FEATURE_LS_CFG_SSBD) &&
> +		    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  			entry->ebx |= F(VIRT_SSBD);
>  		break;
>  	}
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 26110c202b19..950ec50f77c3 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4115,7 +4115,8 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
>  		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))

Shouldn't the IBRS/SSBD check be an "or" check?  I don't think it's
necessarily true that IBRS and SSBD have to both be set.  Maybe something
like:

	if (!msr_info->host_initiated &&
	    !(guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) ||
	      guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))

Does that make sense?

>  			return 1;
>  
>  		msr_info->data = svm->spec_ctrl;
> @@ -4217,11 +4218,12 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  		break;
>  	case MSR_IA32_SPEC_CTRL:
>  		if (!msr->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))

Same question as above.

Thanks,
Tom

>  			return 1;
>  
>  		/* The STIBP bit doesn't fault even if it's not advertised */
> -		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
> +		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD))
>  			return 1;
>  
>  		svm->spec_ctrl = data;
>