linux-kernel - Re: [PATCH] x86/sev: Fix SNP CPUID requests to the hypervisor

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Date:   Mon, 31 Jul 2023 20:58:40 -0700
From:   Dionna Amalie Glaze <dionnaglaze@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michael Roth <michael.roth@....com>,
        Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH] x86/sev: Fix SNP CPUID requests to the hypervisor

> Fixes: ee0bfa08a345 ("x86/compressed/64: Add support for SEV-SNP CPUID table in #VC handlers")
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>

Nice catch.

> +static int __sev_cpuid_hv_ghcb(struct ghcb *ghcb, struct es_em_ctxt *ctxt, struct cpuid_leaf *leaf)
> +{
> +       u32 cr4 = native_read_cr4();
> +       int ret;
> +
> +       ghcb_set_rax(ghcb, leaf->fn);
> +       ghcb_set_rcx(ghcb, leaf->subfn);
> +
> +       if (cr4 & X86_CR4_OSXSAVE)
> +               /* Safe to read xcr0 */
> +               ghcb_set_xcr0(ghcb, xgetbv(XCR_XFEATURE_ENABLED_MASK));
> +       else
> +               /* xgetbv will cause #GP - use reset value for xcr0 */
> +               ghcb_set_xcr0(ghcb, 1);
> +

Everything looks good except I'm confused by this last comment. I
thought xgetbv would #UD if OSXSAVE isn't set in cr4. Is that what
happens after you set it to 1 as some kind of workaround?

....oh I think I do remember hitting this weird technicality when
setting up CPUID support. I think it'd be helpful to document a little
bit more why the ghcb's value is xcr0 is relevant at all when cr4's
OSXSAVE bit is 0 though.


-- 
-Dionna Glaze, PhD (she/her)