KVM: Get writable mapping for __kvm_vcpu_map() only when necessary

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.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-35-seanjc@google.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index baa741c2b81c4cd1b1e88327a17bbae3308daca0..49ac13872e09118a04273f68339cce9c73d7913d 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3116,7 +3116,7 @@ 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,
        };