linux-kernel - Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap

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Date:   Wed, 13 Oct 2021 20:16:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     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 39/45] KVM: SVM: Introduce ops for the post gfn
 map and unmap

On Wed, Oct 13, 2021, Sean Christopherson wrote:
> On Fri, Aug 20, 2021, Brijesh Singh wrote:
> > When SEV-SNP is enabled in the guest VM, the guest memory pages can
> > either be a private or shared. A write from the hypervisor goes through
> > the RMP checks. If hardware sees that hypervisor is attempting to write
> > to a guest private page, then it triggers an RMP violation #PF.
> > 
> > To avoid the RMP violation, add post_{map,unmap}_gfn() ops that can be
> > used to verify that its safe to map a given guest page. Use the SRCU to
> > protect against the page state change for existing mapped pages.
> 
> SRCU isn't protecting anything.  The synchronize_srcu_expedited() in the PSC code
> forces it to wait for existing maps to go away, but it doesn't prevent new maps
> from being created while the actual RMP updates are in-flight.  Most telling is
> that the RMP updates happen _after_ the synchronize_srcu_expedited() call.

Argh, another goof on my part.  Rereading prior feedback, I see that I loosely
suggested SRCU as a possible solution.  That was a bad, bad suggestion.  I think
(hope) I made it offhand without really thinking it through.  SRCU can't work in
this case, because the whole premise of Read-Copy-Update is that there can be
multiple copies of the data.  That simply can't be true for the RMP as hardware
operates on a single table.

In the future, please don't hesitate to push back on and/or question suggestions,
especially those that are made without concrete examples, i.e. are likely off the
cuff.  My goal isn't to set you up for failure :-/