linux-kernel - Re: [PULL 13/19] KVM: SEV: Implement gmem hook for invalidating private pages

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Date: Thu, 16 May 2024 14:45:41 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Michael Roth <michael.roth@....com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PULL 13/19] KVM: SEV: Implement gmem hook for invalidating
 private pages

On Thu, May 16, 2024 at 12:32 AM Sean Christopherson <seanjc@...glecom> wrote:
> > +void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end)
> > +{
> > +     kvm_pfn_t pfn;
> > +
> > +     pr_debug("%s: PFN start 0x%llx PFN end 0x%llx\n", __func__, start, end);
> > +
> > +     for (pfn = start; pfn < end;) {
> > +             bool use_2m_update = false;
> > +             int rc, rmp_level;
> > +             bool assigned;
> > +
> > +             rc = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
> > +             if (WARN_ONCE(rc, "SEV: Failed to retrieve RMP entry for PFN 0x%llx error %d\n",
> > +                           pfn, rc))
> > +                     goto next_pfn;
>
> This is comically trivial to hit, as it fires when running guest_memfd_test on a
> !SNP host.  Presumably the correct fix is to simply do nothing for !sev_snp_guest(),
> but that's easier said than done due to the lack of a @kvm in .gmem_invalidate().
>
> That too is not a big fix, but that's beside the point.  IMO, the fact that I'm
> the first person to (completely inadvertantly) hit this rather basic bug is a
> good hint that we should wait until 6.11 to merge SNP support.

Of course there is an explanation - I usually run all the tests before
pushing anything to kvm/next, here I did not do it because 1) I was
busy with the merge window and 2) I wanted to give exposure to the
code in linux-next, which was the right call indeed but it's beside
the point. Between the clang issue and this one, it's clear that even
though the implementation is 99.99% okay (especially considering the
size), there are a few kinks to fix.

I'll fix everything up and re-push to kvm/next, but I agree that we
shouldn't rush it any further. What really matters is that development
on userspace can proceed.

This also confirms that it's important to replace kvm/next with
kvm/queue in linux-next, since linux-next doesn't care that much about
branches that rebase.

Paolo