Re: [PATCH v2 1/2] x86/cpu, kvm: Use CPU capabilities for CPUID[0x80000021].EAX

From: Borislav Petkov <bp@alien8.de>
To: Kim Phillips <kim.phillips@amd.com>
Cc: x86@kernel.org, Babu Moger <Babu.Moger@amd.com>,
	Borislav Petkov <bp@suse.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Joao Martins <joao.m.martins@oracle.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Juergen Gross <jgross@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Tony Luck <tony.luck@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	kvm@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] x86/cpu, kvm: Use CPU capabilities for CPUID[0x80000021].EAX
Date: Thu, 24 Nov 2022 13:57:54 +0100	[thread overview]
Message-ID: <Y39qUnlRx05eaGeb@zn.tnic> (raw)
In-Reply-To: <20221124000449.79014-2-kim.phillips@amd.com>

On Wed, Nov 23, 2022 at 06:04:48PM -0600, Kim Phillips wrote:
> The AMD Zen4 Automatic IBRS feature bit resides in the 0x80000021 leaf,
> for which there is already support for exposing Zen3 bits to the guest.
> 
> Add AMD AutoIBRS feature bit support, including for the other bits,
> using scattered/synthetic bits.
> 
> Add the corresponding word to KVM's feature machinery so that AutoIBRS
> gets advertized into the guest too.
> 
> Co-developed-by: Babu Moger <Babu.Moger@amd.com>

verify_tags: WARNING: Co-developed-by Babu Moger <Babu.Moger@amd.com> hasn't signed off on the patch!

> Co-developed-by: Borislav Petkov <bp@suse.de>
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>

...

> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index c92c49a0b35b..61cd33a848cc 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -730,6 +730,25 @@ void kvm_set_cpu_caps(void)
>  		0 /* SME */ | F(SEV) | 0 /* VM_PAGE_FLUSH */ | F(SEV_ES) |
>  		F(SME_COHERENT));
>  
> +	/*
> +	 * Pass down these bits:
> +	 *    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
> +	 */
> +	kvm_cpu_cap_init_scattered(CPUID_8000_0021_EAX,
> +				   SF(NO_NESTED_DATA_BP) | SF(LFENCE_RDTSC) |
> +				   SF(NULL_SEL_CLR_BASE) | SF(AUTOIBRS));
> +	if (static_cpu_has(X86_FEATURE_LFENCE_RDTSC))
> +		kvm_cpu_cap_set(X86_FEATURE_LFENCE_RDTSC);
> +	if (!static_cpu_has_bug(X86_BUG_NULL_SEG))
> +		kvm_cpu_cap_set(X86_FEATURE_NULL_SEL_CLR_BASE);

So this looks backwards:

if X86_FEATURE_NULL_SEL_CLR_BASE is set, then X86_BUG_NULL_SEG should
not be.

Which means, you'd have to update check_null_seg_clears_base() too.

Which means, you should make the X86_FEATURE_NULL_SEL_CLR_BASE bit
addition a separate patch because this one is clearly doing too many
things at once.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette