On Mon, Sep 04, 2023 at 10:18:05AM +0200, Claudio Fontana wrote: > 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, It's most likely commit 4b7a6bf402bd ("target-arm: kvm: Differentiate registers based on write-back levels") Thanks, drew > > 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 > >>> > > >
