Re: [PATCH] arm64: Restore trapless ptimer access

[Claudio Fontana]

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,

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
>>>
>