[PATCH v4 01/16] KVM: Allow hva_pfn_fast() to resolve read-only faults.

From: Anish Moorthy <amoorthy@google.com>
To: seanjc@google.com, oliver.upton@linux.dev, kvm@vger.kernel.org,
	kvmarm@lists.linux.dev
Cc: pbonzini@redhat.com, maz@kernel.org, robert.hoo.linux@gmail.com,
	jthoughton@google.com, amoorthy@google.com, bgardon@google.com,
	dmatlack@google.com, ricarkol@google.com,
	axelrasmussen@google.com, peterx@redhat.com,
	nadav.amit@gmail.com, isaku.yamahata@gmail.com
Subject: [PATCH v4 01/16] KVM: Allow hva_pfn_fast() to resolve read-only faults.
Date: Fri,  2 Jun 2023 16:19:06 +0000	[thread overview]
Message-ID: <20230602161921.208564-2-amoorthy@google.com> (raw)
In-Reply-To: <20230602161921.208564-1-amoorthy@google.com>

hva_to_pfn_fast() currently just fails for read-only faults, which is
unnecessary. Instead, try pinning the page without passing FOLL_WRITE.
This allows read-only faults to (potentially) be resolved without
falling back to slow GUP.

Suggested-by: James Houghton <jthoughton@google.com>
Signed-off-by: Anish Moorthy <amoorthy@google.com>
---
 virt/kvm/kvm_main.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index cb5c13eee193..fd80a560378c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2482,7 +2482,7 @@ static inline int check_user_page_hwpoison(unsigned long addr)
 }
 
 /*
- * The fast path to get the writable pfn which will be stored in @pfn,
+ * The fast path to get the pfn which will be stored in @pfn,
  * true indicates success, otherwise false is returned.  It's also the
  * only part that runs if we can in atomic context.
  */
@@ -2496,10 +2496,9 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
 	 * or the caller allows to map a writable pfn for a read fault
 	 * request.
 	 */
-	if (!(write_fault || writable))
-		return false;
+	unsigned int gup_flags = (write_fault || writable) ? FOLL_WRITE : 0;
 
-	if (get_user_page_fast_only(addr, FOLL_WRITE, page)) {
+	if (get_user_page_fast_only(addr, gup_flags, page)) {
 		*pfn = page_to_pfn(page[0]);
 
 		if (writable)
-- 
2.41.0.rc0.172.g3f132b7071-goog

