linux-kernel - Re: [PATCH v10 06/50] x86/sev: Add the host SEV-SNP initialization support

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Date:   Tue, 7 Nov 2023 20:13:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, hpa@...or.com, ardb@...nel.org,
        pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        jmattson@...gle.com, luto@...nel.org, dave.hansen@...ux.intel.com,
        slp@...hat.com, pgonda@...gle.com, peterz@...radead.org,
        srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
        dovmurik@...ux.ibm.com, tobin@....com, vbabka@...e.cz,
        kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
        alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, pankaj.gupta@....com,
        liam.merwick@...cle.com, zhi.a.wang@...el.com,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 06/50] x86/sev: Add the host SEV-SNP initialization
 support

On Tue, Nov 07, 2023 at 12:32:58PM -0600, Tom Lendacky wrote:
> It needs to be called early enough to allow for AutoIBRS to not be disabled
> just because SNP is supported. By calling it where it is currently called,
> the SNP feature can be cleared if, even though supported, SNP can't be used,
> allowing AutoIBRS to be used as a more performant Spectre mitigation.

So far so good.

However, early_rmptable_check -> snp_get_rmptable_info is unnecessary
work which happens on every AP for no reason whatsoever. That's reading
RMP_BASE and RMP_END, doing the same checks which it did on the BSP and
then throwing away the computed rmp_base and rmp_sz, all once per AP.

I don't mind doing early work which needs to be done only once.

I mind doing work which needs to be done only once, on every AP.

Thx.

-- 
Regards/Gruss,
    Boris.

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