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

Sean Christopherson
2024-07-27T00:06:43Z

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 <seanjc@google.com>
---
virt/kvm/kvm_main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

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,
};