linux-kernel - [PATCH gmem] KVM: Relax guest_memfd restrictions on hugepages

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Date:   Mon, 2 Oct 2023 08:33:42 -0500
From:   Michael Roth <michael.roth@....com>
To:     <kvm@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <seanjc@...gle.com>
Subject: [PATCH gmem] KVM: Relax guest_memfd restrictions on hugepages

Rather than requiring an entire memslot's gmem binding to be
hugepage-aligned to make use of hugepages, relax the check to simply
ensure that a large folio is completely contained by the range the
memslot is bound to. Otherwise, userspace components like QEMU may
inadvertantly disable the use of hugepages depending on how they handle
splitting up regions of guest memory for legacy regions, ROMs, etc.

Cc: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Michael Roth <michael.roth@....com>
---
 virt/kvm/guest_memfd.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 4d74b66cfbf7..de5d72e21d63 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -535,6 +535,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
 {
 	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
+	pgoff_t huge_index;
 	struct kvm_gmem *gmem;
 	struct folio *folio;
 	struct page *page;
@@ -574,13 +575,12 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		goto success;
 
 	/*
-	 * For simplicity, allow mapping a hugepage if and only if the entire
-	 * binding is compatible, i.e. don't bother supporting mapping interior
-	 * sub-ranges with hugepages (unless userspace comes up with a *really*
-	 * strong use case for needing hugepages within unaligned bindings).
+	 * Only report the true order of the backing folio if it is fully
+	 * contained by the range this GFN's memslot is bound to.
 	 */
-	if (!IS_ALIGNED(slot->gmem.pgoff, 1ull << *max_order) ||
-	    !IS_ALIGNED(slot->npages, 1ull << *max_order))
+	huge_index = ALIGN(index, 1ull << *max_order);
+	if (huge_index < ALIGN(slot->gmem.pgoff, 1ull << *max_order) ||
+	    huge_index + (1ull << *max_order) > slot->gmem.pgoff + slot->npages)
 		*max_order = 0;
 success:
 	r = 0;
-- 
2.25.1