Due to recent KVM changes, QEMU is setting a ptimer offset resulting in unintended trap and emulate access and a consequent performance hit. Filter out the PTIMER_CNT register to restore trapless ptimer access.
Quoting Andrew Jones: Simply reading the CNT register and writing back the same value is enough to set an offset, since the timer will have certainly moved past whatever value was read by the time it's written. QEMU frequently saves and restores all registers in the get-reg-list array, unless they've been explicitly filtered out (with Linux commit 680232a94c12, KVM_REG_ARM_PTIMER_CNT is now in the array). So, to restore trapless ptimer accesses, we need a QEMU patch to filter out the register. See https://lore.kernel.org/kvmarm/gsntttsonus5....@coltonlewis-kvm.c.googlers.com/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9 for additional context. Signed-off-by: Andrew Jones <andrew.jo...@linux.dev> --- target/arm/kvm64.c | 1 + 1 file changed, 1 insertion(+) diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c index 4d904a1d11..2dd46e0a99 100644 --- a/target/arm/kvm64.c +++ b/target/arm/kvm64.c @@ -672,6 +672,7 @@ typedef struct CPRegStateLevel { */ static const CPRegStateLevel non_runtime_cpregs[] = { { KVM_REG_ARM_TIMER_CNT, KVM_PUT_FULL_STATE }, + { KVM_REG_ARM_PTIMER_CNT, KVM_PUT_FULL_STATE }, }; int kvm_arm_cpreg_level(uint64_t regidx) -- 2.42.0.283.g2d96d420d3-goog
