linux-kernel - Re: [PATCH v9 22/43] x86/sev: Move MSR-based VMGEXITs for CPUID to helper

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Date:   Thu, 3 Feb 2022 14:59:36 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v9 22/43] x86/sev: Move MSR-based VMGEXITs for CPUID to
 helper

On Fri, Jan 28, 2022 at 11:17:43AM -0600, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@....com>
> 
> This code will also be used later for SEV-SNP-validated CPUID code in
> some cases, so move it to a common helper.

Suggested-by: Sean Christopherson <seanjc@...gle.com>

unless he really doesn't want to be the "suggestor" :)

> Signed-off-by: Michael Roth <michael.roth@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/kernel/sev-shared.c | 62 +++++++++++++++++++++---------------
>  1 file changed, 36 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 3aaef1a18ffe..633f1f93b6e1 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -194,6 +194,36 @@ enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb, bool set_ghcb_msr,
>  	return verify_exception_info(ghcb, ctxt);
>  }
>  
> +static int __sev_cpuid_hv(u32 func, int reg_idx, u32 *reg)
> +{
> +	u64 val;
> +
> +	if (!reg)
> +		return 0;

I don't know what that's supposed to catch? In case callers are
interested only in some subset of the CPUID leaf?

Meh, I guess...

> +	sev_es_wr_ghcb_msr(GHCB_CPUID_REQ(func, reg_idx));
> +	VMGEXIT();
> +	val = sev_es_rd_ghcb_msr();
> +	if (GHCB_RESP_CODE(val) != GHCB_MSR_CPUID_RESP)
> +		return -EIO;
> +
> +	*reg = (val >> 32);
> +
> +	return 0;
> +}
> +
> +static int sev_cpuid_hv(u32 func, u32 *eax, u32 *ebx, u32 *ecx, u32 *edx)
> +{
> +	int ret;
> +
> +	ret = __sev_cpuid_hv(func, GHCB_CPUID_REQ_EAX, eax);
> +	ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EBX, ebx);
> +	ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_ECX, ecx);
> +	ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EDX, edx);

You can format that this way:

        ret =         __sev_cpuid_hv(func, GHCB_CPUID_REQ_EAX, eax);
        ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EBX, ebx);
        ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_ECX, ecx);
        ret = ret ? : __sev_cpuid_hv(func, GHCB_CPUID_REQ_EDX, edx);


and then it is visible at a quick glance what this does, due to the
regularity.

-- 
Regards/Gruss,
    Boris.

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