linux-kernel - Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under virtualization

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Date:   Sun, 6 Nov 2022 09:38:56 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Kim Phillips <kim.phillips@....com>, x86@...nel.org
Cc:     Borislav Petkov <bp@...en8.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Babu Moger <Babu.Moger@....com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] x86/speculation: Support Automatic IBRS under
 virtualization

On 11/4/22 22:36, Kim Phillips wrote:
> @@ -730,6 +730,8 @@ void kvm_set_cpu_caps(void)
>   		0 /* SME */ | F(SEV) | 0 /* VM_PAGE_FLUSH */ | F(SEV_ES) |
>   		F(SME_COHERENT));
>   
> +	kvm_cpu_cap_mask(CPUID_8000_0021_EAX, F(AUTOIBRS));

This should also include bits 0, 2 and 6.  Feel free to add #defines for
them in cpuid.c if x86 maintainers do not want them in cpufeatures.h.

There should also be something like:

                 if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
                         kvm_cpu_cap_set(CPUID_8000_0021_EAX, F(AMD_LFENCE_RDTSC));
                 if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
                         kvm_cpu_cap_set(CPUID_8000_0021_EAX, F(NSCB);

so that...

> @@ -1211,12 +1213,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		 *    EAX      0      NNDBP, Processor ignores nested data breakpoints
>  		 *    EAX      2      LAS, LFENCE always serializing
>  		 *    EAX      6      NSCB, Null selector clear base
> +		 *    EAX      8      Automatic IBRS
>  		 *
>  		 * Other defined bits are for MSRs that KVM does not expose:
>  		 *   EAX      3      SPCL, SMM page configuration lock
>  		 *   EAX      13     PCMSR, Prefetch control MSR
>  		 */
> -		entry->eax &= BIT(0) | BIT(2) | BIT(6);
> +		entry->eax &= BIT(0) | BIT(2) | BIT(6) | BIT(8);
>  		if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
>  			entry->eax |= BIT(2);
>  		if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
>  			entry->eax |= BIT(6);

... these five lines become simply

	cpuid_entry_override(entry, CPUID_8000_0021_EAX);

In the end these should be two patches:

- kvm, x86: use CPU capabilities for CPUID[0x80000021].EAX
- kvm, x86: support AMD automatic IBRS

Thanks,

Paolo