linux-kernel - Re: [PATCH Part2 v5 34/45] KVM: SVM: Do not use long-lived GHCB map while setting scratch area

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Date:   Fri, 15 Oct 2021 11:11:19 -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-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>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 34/45] KVM: SVM: Do not use long-lived GHCB map
 while setting scratch area


On 10/13/21 2:20 PM, Sean Christopherson wrote:
> On Fri, Aug 20, 2021, Brijesh Singh wrote:
>> The setup_vmgexit_scratch() function may rely on a long-lived GHCB
>> mapping if the GHCB shared buffer area was used for the scratch area.
>> In preparation for eliminating the long-lived GHCB mapping, always
>> allocate a buffer for the scratch area so it can be accessed without
>> the GHCB mapping.
> Would it make sense to post this patch and the next (Remove the long-lived GHCB
> host map) in a separate mini-series?  It's needed for SNP, but AFAICT there's
> nothing that depends on SNP.  Getting this merged ahead of time would reduce the
> size of the SNP series by a smidge.

While testing with random configs, I am seeing some might_sleep() warns.
This is happening mainly because during the vmrun the GHCB is accessed
with preempt disabled. The kvm_vcpu_map() -> kmap() reports the warning.
I am leaning towards creating a cache on the vmgexit and use that cache
instead of the doing a kmap() on every access. Does that sound okay to you ?

-Brijesh