[PATCH] KVM: riscv: Set Accessed bit in dirty-log write fault fast path

[PATCH] KVM: riscv: Set Accessed bit in dirty-log write fault fast path

[SeungJu Cheon]

kvm_riscv_mmu_dirty_log_write_fault_fast() considers a leaf with W
and D set already handled, even if aging has cleared its A bit.
On hardware that faults when A is clear, the fast path returns
true without setting A and skips the slow path that would set it.
The guest retries the same store, but A is still clear, so it
faults again. This repeats indefinitely, preventing the guest
from making progress.

Host reclaim or page_idle can trigger this while dirty logging is
enabled, potentially hanging vCPUs during live migration.

Require _PAGE_ACCESSED in addition to W and D before treating the
fault as handled, and set A along with W and D. The existing
cmpxchg loop retries if aging changes the PTE before the update.

Fixes: 7705be59eb2d ("KVM: riscv: Fast-path dirty logging write faults")
Cc: stable@vger.kernel.org # 7.2+
Signed-off-by: SeungJu Cheon <suunj1331@gmail.com>
---
Testing:

On a HiFive Premier P550 (EIC7700X, which traps on clear A/D bits),
run:

dirty_log_perf_test -v 3 -b 256M -i 3

While the test runs, repeatedly mark all system RAM idle through
/sys/kernel/mm/page_idle/bitmap.

Before the fix, the test did not complete its first iteration in
over 11 minutes, with lock_stat showing about 16k read-side
mmu_lock acquisitions per second from the fast path.

After the fix, all three iterations complete, each taking about
29 seconds under the same aging workload.

 arch/riscv/kvm/mmu.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
index 78a1826c466d..042d1e3b3547 100644
--- a/arch/riscv/kvm/mmu.c
+++ b/arch/riscv/kvm/mmu.c
@@ -591,14 +591,15 @@ static bool kvm_riscv_mmu_dirty_log_write_fault_fast(struct kvm *kvm,
 			dirty_marked = true;
 		}
 
-		if ((pte_val(old_pte) & (_PAGE_WRITE | _PAGE_DIRTY)) ==
-		    (_PAGE_WRITE | _PAGE_DIRTY)) {
+		if ((pte_val(old_pte) &
+		     (_PAGE_WRITE | _PAGE_DIRTY | _PAGE_ACCESSED)) ==
+		    (_PAGE_WRITE | _PAGE_DIRTY | _PAGE_ACCESSED)) {
 			new_pte = old_pte;
 			ret = true;
 			break;
 		}
 
-		new_pte = pte_mkdirty(pte_mkwrite_novma(old_pte));
+		new_pte = pte_mkyoung(pte_mkdirty(pte_mkwrite_novma(old_pte)));
 
 		if (kvm_riscv_gstage_try_update_pte(&gstage, ptep_level, gpa,
 						    ptep, old_pte, new_pte)) {
-- 
2.52.0