[v12,32/84] KVM: Get writable mapping for __kvm_vcpu_map() only when necessary

Commit Message

Sean Christopherson July 26, 2024, 11:51 p.m. UTC

When creating a memory map for read, don't request a writable pfn from the
primary MMU.  While creating read-only mappings can be theoretically slower,
as they don't play nice with fast GUP due to the need to break CoW before
mapping the underlying PFN, practically speaking, creating a mapping isn't
a super hot path, and getting a writable mapping for reading is weird and
confusing.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a46c7bf1f902..a28479629488 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3142,7 +3142,7 @@  int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
 	struct kvm_follow_pfn kfp = {
 		.slot = gfn_to_memslot(vcpu->kvm, gfn),
 		.gfn = gfn,
-		.flags = FOLL_WRITE,
+		.flags = writable ? FOLL_WRITE : 0,
 		.refcounted_page = &map->pinned_page,
 		.pin = true,
 	};