On Mon, Oct 21, 2024, Yan Zhao wrote:

On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote:

quoted

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 <redacted>
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 080740f65061..b845e9252633 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c

@@ -3122,7 +3122,7 @@ 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,
 	};

When writable is false, could we set ".pin = false," ?

Hmm, maybe?  I can't imagine anything would actually break, but unless FOLL_PIN
implies writing, my preference would still be to pin the page so that KVM always
pins when accessing the actual data of a page.

Also not sure if ".map_writable = NULL" is missing.

Doh, my previous response was slightly wrong, it's implicitly initialized to NULL,
not false.  I forgot map_writable is a pointer to a bool.