linux-kernel - Re: [PATCH v2 2/2] KVM: arm64: timers: Save restore CVAL of a ptimer across guest entry and exits

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Message-ID: <ff458bae-2681-1464-d68e-f9d04098c8b4@os.amperecomputing.com>
Date:   Mon, 25 Sep 2023 11:10:18 +0530
From:   Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, Christoffer.Dall@....com,
        eauger@...hat.com, miguel.luis@...cle.com,
        darren@...amperecomputing.com, scott@...amperecomputing.com
Subject: Re: [PATCH v2 2/2] KVM: arm64: timers: Save restore CVAL of a ptimer
 across guest entry and exits



On 25-09-2023 11:05 am, Ganapatrao Kulkarni wrote:
> 
> 
> On 24-09-2023 03:18 pm, Marc Zyngier wrote:
>> On Tue, 19 Sep 2023 07:15:44 +0100,
>> Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com> wrote:
>>>
>>>
>>>
>>> On 18-09-2023 04:59 pm, Marc Zyngier wrote:
>>>> On Fri, 15 Sep 2023 10:57:46 +0100,
>>>> Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com> wrote:
>>>>>
>>>>> This patch did not work.
>>>>> After adding changes as in below diff, it is started working.
>>>>
>>>> Thanks for looking into this.
>>>>
>>>>>
>>>>> diff --git a/arch/arm64/kvm/hyp/vhe/switch.c
>>>>> b/arch/arm64/kvm/hyp/vhe/switch.c
>>>>> index b0b07658f77d..91d2cfb03e26 100644
>>>>> --- a/arch/arm64/kvm/hyp/vhe/switch.c
>>>>> +++ b/arch/arm64/kvm/hyp/vhe/switch.c
>>>>> @@ -117,7 +117,7 @@ static void __activate_traps(struct kvm_vcpu 
>>>>> *vcpu)
>>>>>                           val = __vcpu_sys_reg(vcpu, CNTHP_CVAL_EL2);
>>>>>
>>>>>                   if (map.direct_ptimer) {
>>>>> -                       write_sysreg_s(val, SYS_CNTP_CVAL_EL0);
>>>>> +                       write_sysreg_el0(val, SYS_CNTP_CVAL);
>>>>
>>>> Duh, of course. Silly me.
>>>>
>>>>>                           isb();
>>>>>                   }
>>>>>           }
>>>>> @@ -161,8 +161,6 @@ static void __deactivate_traps(struct kvm_vcpu 
>>>>> *vcpu)
>>>>>
>>>>>           ___deactivate_traps(vcpu);
>>>>>
>>>>> -       write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
>>>>> -
>>>>>           if (has_cntpoff()) {
>>>>>                   struct timer_map map;
>>>>>                   u64 val, offset;
>>>>> @@ -173,7 +171,7 @@ static void __deactivate_traps(struct kvm_vcpu 
>>>>> *vcpu)
>>>>>                    * We're exiting the guest. Save the latest CVAL 
>>>>> value
>>>>>                    * to memory and apply the offset now that TGE is 
>>>>> set.
>>>>>                    */
>>>>> -               val = read_sysreg_s(SYS_CNTP_CVAL_EL0);
>>>>> +               val = read_sysreg_el0(SYS_CNTP_CVAL);
>>>>>                   if (map.direct_ptimer == vcpu_ptimer(vcpu))
>>>>>                           __vcpu_sys_reg(vcpu, CNTP_CVAL_EL0) = val;
>>>>>                   if (map.direct_ptimer == vcpu_hptimer(vcpu))
>>>>> @@ -182,12 +180,13 @@ static void __deactivate_traps(struct 
>>>>> kvm_vcpu *vcpu)
>>>>>                   offset = read_sysreg_s(SYS_CNTPOFF_EL2);
>>>>>
>>>>>                   if (map.direct_ptimer && offset) {
>>>>> -                       offset = read_sysreg_s(SYS_CNTPOFF_EL2);
>>>>> -                       write_sysreg_s(val + offset, 
>>>>> SYS_CNTP_CVAL_EL0);
>>>>> +                       write_sysreg_el0(val + offset, SYS_CNTP_CVAL);
>>>>>                           isb();
>>>>>                   }
>>>>>           }
>>>>>
>>>>> +       write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
>>>>
>>>> Why moving the HCR_EL2 update? I don't grok what it changes. Or is it
>>>
>>> This the line of code which flips the TGE and making timer cval ready
>>> to handle the TGE flip is more safe way(avoids even corner case of
>>> false interrupt triggers) than changing after the flipping?
>>
>> That's pretty dubious. Do you actually see it firing on your HW?
>>
>>>
>>>> that you end-up with spurious interrupts because your GIC is slow to
>>>> retire interrupts that are transiently pending?
>>>
>>> IIUC, If there are any transient interrupts or asserted already,
>>> anyway they will be handled when irq is unmasked.
>>
>> That's the idea. But my question is whether you observe spurious
>> interrupts when not changing the ordering.
> 
> I tried with keeping the ordering (i.e flip TGE then change cval) and i 
> don't see any issue as such. IMO, it is better to have cval updated 
> before TGE flip, anyway either way works for us.
> 

Please feel free to add,
Tested-by: Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>

>>
>> Thanks,
>>
>>     M.
>>
> 
> Thanks,
> Ganapat

Thanks,
Ganapat