linux-kernel - Re: [PATCH] arch/x86: Set XSS while handling #VC intercept for CPUID

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Message-ID: <e7ae2b89-f2c4-e95f-342b-fcf92a2e0ae3@intel.com>
Date:   Tue, 3 Oct 2023 09:07:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jinank Jain <jinankjain@...ux.microsoft.com>, seanjc@...gle.com,
        pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, jinankjain@...rosoft.com, thomas.lendacky@....com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     wei.liu@...nel.org, tiala@...rosoft.com
Subject: Re: [PATCH] arch/x86: Set XSS while handling #VC intercept for CPUID

On 10/3/23 02:28, Jinank Jain wrote:
...
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 2eabccde94fb..92350a24848c 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -880,6 +880,9 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb,
>  	if (snp_cpuid_ret != -EOPNOTSUPP)
>  		return ES_VMM_ERROR;
>  
> +	if (regs->ax == 0xD && regs->cx == 0x1)
> +		ghcb_set_xss(ghcb, 0);

The spec talks about leaf 0xD, but not the subleaf:

> XSS is only required to besupplied when a request forCPUID 0000_000D
> is made andthe guest supports the XSS MSR(0x0000_0DA0).
Why restrict this to subleaf (regx->cx) 1?

Second, XCR0 is being supplied regardless of the CPUID leaf.  Why should
XSS be restricted to 0xD while XCR0 is universally supplied?

Third, why is it OK to supply a garbage (0) value?  If the GHCB field is
required it's surely because the host *NEEDS* the value to do something.
 Won't a garbage value potentially confuse the host?