linux-kernel - Re: [RFC PATCH v2] x86/sev-es: Include XSS value in GHCB CPUID request

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Message-ID: <20230615115255.GBZIr7l2XNKau16ayl@fat_crate.local>
Date:   Thu, 15 Jun 2023 13:52:55 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     John Allen <john.allen@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, weijiang.yang@...el.com,
        rick.p.edgecombe@...el.com, seanjc@...gle.com, x86@...nel.org,
        thomas.lendacky@....com
Subject: Re: [RFC PATCH v2] x86/sev-es: Include XSS value in GHCB CPUID
 request

On Wed, May 24, 2023 at 03:56:19PM +0000, John Allen wrote:
> +	if (has_cpuflag(X86_FEATURE_SHSTK) && regs->ax == 0xd && regs->cx <= 1) {
> +		unsigned long lo, hi;
> +		u64 xss;
> +
> +		/*
> +		 * Since vc_handle_cpuid may be used during early boot, the
> +		 * rdmsr wrappers are incompatible and should not be used.
> +		 * Invoke the instruction directly.
> +		 */
> +		asm volatile("rdmsr" : "=a" (lo), "=d" (hi)
> +			     : "c" (MSR_IA32_XSS));
> +		xss = (hi << 32) | lo;
> +		ghcb_set_xss(ghcb, xss);

$ git grep ghcb_set_xss
$

So this patch needs some tree which I'm not aware of.

Also, this passing through of host XSS to the guest looks like it is
bypassing the vcpu->arch.ia32_xss copy which KVM seems to maintain. It
looks to me like the handling needs to be synchronized with it or so.

Thx.

-- 
Regards/Gruss,
    Boris.

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