Re: [PATCH] arm64: Restore trapless ptimer access

[Claudio Fontana]

On 9/4/23 13:07, Andrew Jones wrote:
> 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@arm.com/
>>
>> 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")

Indeed, thanks!

C

> 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@coltonlewis-kvm.c.googlers.com/T/#m0770023762a821db2a3f0dd0a7dc6aa54e0d0da9
>>>>
>>>> The link can be shorter with
>>>>
>>>> https://lore.kernel.org/all/20230823200408.1214332-1-coltonlewis@google.com/
>>>
>>> I will keep that in mind next time.
>>>
>>>>> for additional context.
>>>>>
>>>>> Signed-off-by: Andrew Jones <andrew.jones@linux.dev>
>>>>
>>>> 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@google.com>
>>> Tested-by: Colton Lewis <coltonlewis@google.com>
>>>
>>>>> ---
>>>>>  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
>>>>>
>>>
>>