On Fri, Sep 01, 2023 at 09:35:47AM +0200, Andrew Jones wrote: > 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/ I will keep that in mind next time. > > 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. Assuming it is sufficient to add here instead of reposting the whole patch: Signed-off-by: Colton Lewis <coltonlewis@xxxxxxxxxx> Tested-by: Colton Lewis <coltonlewis@xxxxxxxxxx> > > --- > > 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 > >
