linux-kernel - Re: [PATCH v12 22/29] KVM: SEV: Implement gmem hook for invalidating private pages

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Date: Thu, 18 Apr 2024 14:57:54 -0500
From: Michael Roth <michael.roth@....com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
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Subject: Re: [PATCH v12 22/29] KVM: SEV: Implement gmem hook for invalidating
 private pages

On Sat, Mar 30, 2024 at 10:31:47PM +0100, Paolo Bonzini wrote:
> On 3/29/24 23:58, Michael Roth wrote:
> > +		/*
> > +		 * If an unaligned PFN corresponds to a 2M region assigned as a
> > +		 * large page in he RMP table, PSMASH the region into individual
> > +		 * 4K RMP entries before attempting to convert a 4K sub-page.
> > +		 */
> > +		if (!use_2m_update && rmp_level > PG_LEVEL_4K) {
> > +			rc = snp_rmptable_psmash(pfn);
> > +			if (rc)
> > +				pr_err_ratelimited("SEV: Failed to PSMASH RMP entry for PFN 0x%llx error %d\n",
> > +						   pfn, rc);
> > +		}
> 
> Ignoring the PSMASH failure is pretty scary...  At this point .free_folio
> cannot fail, should the psmash part of this patch be done in
> kvm_gmem_invalidate_begin() before kvm_mmu_unmap_gfn_range()?
> 
> Also, can you get PSMASH_FAIL_INUSE and if so what's the best way to address
> it?  Should fallocate() return -EBUSY?

FAIL_INUSE shouldn't occur since at this point the pages have been unmapped
from NPT and only the task doing the cleanup should be attempting to
access/PSMASH this particular 2M HPA range at this point.

However, since FAIL_INUSE is transient, there isn't a good reason why we
shouldn't retry until it clears itself up rather than risk hosing the
system if some unexpected case ever did pop up, so I've updated
snp_rmptable_psmash() to handle that case automatically and simplify the
handling in sev_handle_rmp_fault() as well. (in the case of #NPF RMP
faults there is actually potential for PSMASH errors other than
FAIL_INUSE due to races with other vCPU threads which can interleave and
put the RMP entry in an unexpected state, so there's additional
handling/reporting to deal with those cases, but here they are not expected
and will trigger WARN_*ONCE()'s now)

I used this hacked up version of Sean's original patch to re-enable 2MB
hugepage support in gmem for the purposes of re-testing this:

  https://github.com/mdroth/linux/commit/15aa4f81811485997953130fc184e829ba4399d2

-Mike

> 
> Thanks,
> 
> Paolo
> 
>