On Thu, Aug 31, 2023 at 07:00:52PM +0000, Colton Lewis wrote: > 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.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9 The link can be shorter with https://lore.kernel.org/all/20230823200408.1214332-1-coltonlewis@xxxxxxxxxx/ > for additional context. > > Signed-off-by: Andrew Jones <andrew.jones@xxxxxxxxx> Thanks for the testing and posting, Colton. Please add your s-o-b and a Tested-by tag as well. Thanks, drew > --- > 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 >
