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

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Date: Wed, 15 May 2024 15:32:31 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: Paolo Bonzini <pbonzini@...hat.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 Fri, May 10, 2024, Michael Roth wrote:
> Implement a platform hook to do the work of restoring the direct map
> entries of gmem-managed pages and transitioning the corresponding RMP
> table entries back to the default shared/hypervisor-owned state.

..

> +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.