On Wed, Jul 05, 2023, Yu Zhang wrote:

On Tue, Jul 04, 2023 at 04:50:49PM +0900, David Stevens wrote:

quoted

From: David Stevens <redacted>

Migrate from __gfn_to_pfn_memslot to __kvm_follow_pfn.

Please turn up your changelog verbosity from ~2 to ~8.  E.g. explain the transition
from async => FOLL_NOWAIT+KVM_PFN_ERR_NEEDS_IO, there's no reason to force readers
to suss that out on their own.

quoted

Signed-off-by: David Stevens <redacted>
---
 arch/x86/kvm/mmu/mmu.c | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index ec169f5c7dce..e44ab512c3a1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c

@@ -4296,7 +4296,12 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
 static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	struct kvm_memory_slot *slot = fault->slot;
-	bool async;
+	struct kvm_follow_pfn foll = {
+		.slot = slot,
+		.gfn = fault->gfn,
+		.flags = FOLL_GET | (fault->write ? FOLL_WRITE : 0),
+		.allow_write_mapping = true,
+	};
 
 	/*
 	 * Retry the page fault if the gfn hit a memslot that is being deleted

@@ -4325,12 +4330,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 			return RET_PF_EMULATE;
 	}
 
-	async = false;
-	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
-					  fault->write, &fault->map_writable,
-					  &fault->hva);
-	if (!async)
-		return RET_PF_CONTINUE; /* *pfn has correct page already */
+	foll.flags |= FOLL_NOWAIT;
+	fault->pfn = __kvm_follow_pfn(&foll);
+
+	if (!is_error_noslot_pfn(fault->pfn))
+		goto success;
+
+	if (fault->pfn != KVM_PFN_ERR_NEEDS_IO)
+		return RET_PF_CONTINUE;

IIUC, FOLL_NOWAIT is set only when we wanna an async fault. So
KVM_PFN_ERR_NEEDS_IO may not be necessary? 

But FOLL_NOWAIT is set above.  This logic is essentially saying "bail immediately
if __gfn_to_pfn_memslot() returned a fatal error".

A commented would definitely be helpful though.  How about?

	/*
	 * If __kvm_follow_pfn() failed because I/O is needed to fault in the
	 * page, then either set up an asynchronous #PF to do the I/O, or if
	 * doing an async #PF isn't possible, retry __kvm_follow_pfn() with
	  I/O allowed. All other failures are fatal, i.e. retrying won't help.
	 */
	if (fault->pfn != KVM_PFN_ERR_NEEDS_IO)
		return RET_PF_CONTINUE;