linux-kernel - [PATCH RFC v1 4/5] KVM: guest_memfd: Don't prepare shared folios

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Message-ID: <20250613005400.3694904-5-michael.roth@amd.com>
Date: Thu, 12 Jun 2025 19:53:59 -0500
From: Michael Roth <michael.roth@....com>
To: <kvm@...r.kernel.org>
CC: <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <david@...hat.com>, <tabba@...gle.com>,
	<vannapurve@...gle.com>, <ackerleytng@...gle.com>, <ira.weiny@...el.com>,
	<thomas.lendacky@....com>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<vbabka@...e.cz>, <joro@...tes.org>, <pratikrajesh.sampat@....com>,
	<liam.merwick@...cle.com>, <yan.y.zhao@...el.com>, <aik@....com>
Subject: [PATCH RFC v1 4/5] KVM: guest_memfd: Don't prepare shared folios

In the current guest_memfd logic, "preparation" is only used currently
to describe the additional work of putting a guest_memfd page into an
architecturally-defined "private" state, such as updating RMP table
entries for SEV-SNP guests. As such, there's no input to the
corresponding kvm_arch_gmem_prepare() hooks as to whether a page is
being prepared/accessed as shared or as private, so "preparation" will
end up being erroneously done on pages that were supposed to remain in a
shared state. Rather than plumb through the additional information
needed to distinguish between shared vs. private preparation, just
continue to only do preparation on private pages, as was the case prior
to support for GUEST_MEMFD_FLAG_SUPPORT_SHARED being introduced.

Signed-off-by: Michael Roth <michael.roth@....com>
---
 virt/kvm/guest_memfd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index f27e1f3962bb..a912b00776f1 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -1376,7 +1376,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		folio_mark_uptodate(folio);
 	}

-	r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
+	if (kvm_gmem_shareability_get(file_inode(file), index) == SHAREABILITY_GUEST)
+		r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);

 	folio_unlock(folio);

-- 
2.25.1