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

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Message-ID: <20231107191931.GCZUqNwxP8JcSbjZ0/@fat_crate.local>
Date:   Tue, 7 Nov 2023 20:19:31 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <ashish.kalra@....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, thomas.lendacky@....com, 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, 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 01:00:00PM -0600, Kalra, Ashish wrote:
> > First of all, use the APM bit name here pls: MtrrFixDramModEn.
> > 
> > And then, for the life of me, I can't find any mention in the APM why
> > this bit is needed. Neither in "15.36.2 Enabling SEV-SNP" nor in
> > "15.34.3 Enabling SEV".
> > 
> > Looking at the bit defintions of WrMem an RdMem - read and write
> > requests get directed to system memory instead of MMIO so I guess you
> > don't want to be able to write MMIO for certain physical ranges when SNP
> > is enabled but it'll be good to have this properly explained instead of
> > a "this must happen" information-less sentence.
> 
> This is a per-requisite for SNP_INIT as per the SNP Firmware ABI
> specifications, section 8.8.2:

Did you even read the text you're responding to?

> > This looks backwards. AFAICT, the IOMMU code should call arch code to
> > enable SNP at the right time, not the other way around - arch code
> > calling driver code.
> > 
> > Especially if the SNP table enablement depends on some exact IOMMU
> > init_state:
> > 
> >          if (init_state > IOMMU_ENABLED) {
> > 		pr_err("SNP: Too late to enable SNP for IOMMU.\n");
> > 
> > 
> 
> This is again as per SNP_INIT requirements, that SNP support is enabled in
> the IOMMU before SNP_INIT is done. The above function snp_rmptable_init()
> only calls the IOMMU driver to enable SNP support when it has detected and
> enabled platform support for SNP.
>v
> It is not that IOMMU driver has to call the arch code to enable SNP at the
> right time but it is the other way around that once platform support for SNP
> is enabled then the IOMMU driver has to be called to enable the same for the
> IOMMU and this needs to be done before the CCP driver is loaded and does
> SNP_INIT.

You again didn't read the text you're responding to.

Arch code does not call drivers - arch code sets up the arch and
provides facilities which the drivers use.

-- 
Regards/Gruss,
    Boris.

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