linux-kernel - Re: [PATCH v7 10/45] x86/sev: Add support for hypervisor feature VMGEXIT

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Date:   Thu, 2 Dec 2021 18:52:15 +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>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v7 10/45] x86/sev: Add support for hypervisor feature
 VMGEXIT

On Wed, Nov 10, 2021 at 04:06:56PM -0600, Brijesh Singh wrote:
> +/*
> + * The hypervisor features are available from GHCB version 2 onward.
> + */
> +static bool get_hv_features(void)
> +{
> +	u64 val;
> +
> +	sev_hv_features = 0;
> +
> +	if (ghcb_version < 2)
> +		return false;
> +
> +	sev_es_wr_ghcb_msr(GHCB_MSR_HV_FT_REQ);
> +	VMGEXIT();
> +
> +	val = sev_es_rd_ghcb_msr();
> +	if (GHCB_RESP_CODE(val) != GHCB_MSR_HV_FT_RESP)
> +		return false;
> +
> +	sev_hv_features = GHCB_MSR_HV_FT_RESP_VAL(val);
> +
> +	return true;
> +}

I still don't like this.

This is more of that run-me-in-the-exception-handler thing while this is
purely feature detection stuff which needs to be done exactly once on
init.

IOW, that stanza

        if (!sev_es_negotiate_protocol())
                sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_PROT_UNSUPPORTED);

should be called once in sev_enable() for the decompressor kernel and
once in sev_es_init_vc_handling() for kernel proper.

Then you don't need to do any of that sev_hv_features = 0 thing but
detect them exactly once and query them as much as you can.

Thx.

-- 
Regards/Gruss,
    Boris.

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