Hi, I think this discussion from ~2015 could potentially be be historically relevant for context, at the time we had the problem with CNTVOFF IIRC so KVM_REG_ARM_TIMER_CNT being read and rewritten causing time warps in the guest: https://patchwork.kernel.org/project/linux-arm-kernel/patch/1435157697-28579-1-git-send-email-marc.zyngier@xxxxxxx/ I could not remember or find if/where the problem was fixed in the end in QEMU, Ciao, Claudio On 9/1/23 21:23, Colton Lewis wrote: > 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 >>> >
