linux-kernel - Re: [PATCH Part2 RFC v4 33/40] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

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Date:   Mon, 19 Jul 2021 14:15:53 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, 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,
        linux-crypto@...r.kernel.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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 33/40] KVM: SVM: Add support to handle MSR
 based Page State Change VMGEXIT



On 7/19/21 1:55 PM, Sean Christopherson wrote:
> 
> I've no objection to using PSC for enums and whatnot, and I'll happily defer to
> Boris for functions in the core kernel and guest, but for KVM I'd really like to
> spell out the name for the two or so main handler functions.

Noted.


>>
>> Maybe I am missing something, I am not able to follow 'guest won't be able
>> to convert a 2mb chunk back to a 2mb large page'. The page-size used inside
>> the guest have to relationship with the RMP/NPT page-size. e.g, a guest can
>> validate the page range as a 4k and still map the page range as a 2mb or 1gb
>> in its pagetable.
> 
> The proposed code walks KVM's TDP and adjusts the RMP level to be the min of the
> guest+host levels.  Once KVM has installed a 4kb TDP SPTE, that walk will find
> the 4kb TDP SPTE and thus operate on the RMP at a 4kb granularity.  To allow full
> restoration of 2mb PTE+SPTE+RMP, KVM needs to zap the 4kb SPTE(s) at some point
> to allow rebuilding a 2mb SPTE.
> 

Ah I see. In that case, SNP firmware provides a command 
"SNP_PAGE_UNMASH" that can be used by the hypervisor to combines the 
multiple 4k entry into a single 2mb without affecting the validation.

-Brijesh