Re: [PATCH] ArmPkg/TimerDxe: remove workaround for KVM timer handling

[Laszlo Ersek <lersek@redhat.com>]

On 03/15/18 20:01, Leif Lindholm wrote:
> On Thu, Mar 15, 2018 at 10:28:26AM +0000, Ard Biesheuvel wrote:
>> When we first ported EDK2 to KVM/arm, we implemented a workaround for
>> the quirky timer handling on the KVM side. This has been fixed in
>> Linux commit f120cd6533d2 ("KVM: arm/arm64: timer: Allow the timer to
>> control the active state") dated 23 June 2014, which was incorporated
>> into Linux release 4.3.
>>
>> So almost 4 years later, it should be safe to drop this workaround on
>> the EDK2 side.
>>
>> This reverts commit b1a633434ddc.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> 
> I'm happy with this, with Marc's Ack.
> Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
> 
> However, if this can affect old kernels running in vms, could you ping
> cross-distro@lists.linaro.org as well, so it doesn't catch anyone by
> surprise?

If I understand correctly, the patch could negatively affect old kernels
that run on *hosts*. (Discounting nested virtualization, which I'm
perfectly willing to ignore for this discussion.)

Emailing the cross-distro list shouldn't hurt in any case.

>From my side (since I was CC'd):

Acked-by: Laszlo Ersek <lersek@redhat.com>

Thanks!
Laszlo

> 
> /
>     Leif
> 
>> ---
>>  ArmPkg/Drivers/TimerDxe/TimerDxe.c                                           |  1 -
>>  ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c | 10 ----------
>>  2 files changed, 11 deletions(-)
>>
>> diff --git a/ArmPkg/Drivers/TimerDxe/TimerDxe.c b/ArmPkg/Drivers/TimerDxe/TimerDxe.c
>> index a3202fa056f3..bd616d2efc73 100644
>> --- a/ArmPkg/Drivers/TimerDxe/TimerDxe.c
>> +++ b/ArmPkg/Drivers/TimerDxe/TimerDxe.c
>> @@ -337,7 +337,6 @@ TimerInterruptHandler (
>>  
>>      // Set next compare value
>>      ArmGenericTimerSetCompareVal (CompareValue);
>> -    ArmGenericTimerEnableTimer ();
>>      ArmInstructionSynchronizationBarrier ();
>>    }
>>  
>> diff --git a/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c b/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
>> index 69a4ceb62db6..c941895a3574 100644
>> --- a/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
>> +++ b/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
>> @@ -26,16 +26,6 @@ ArmGenericTimerEnableTimer (
>>  
>>    TimerCtrlReg = ArmReadCntvCtl ();
>>    TimerCtrlReg |= ARM_ARCH_TIMER_ENABLE;
>> -
>> -  //
>> -  // When running under KVM, we need to unmask the interrupt on the timer side
>> -  // as KVM will mask it when servicing the interrupt at the hypervisor level
>> -  // and delivering the virtual timer interrupt to the guest. Otherwise, the
>> -  // interrupt will fire again, trapping into the hypervisor again, etc. etc.
>> -  // This is scheduled to be fixed on the KVM side, but there is no harm in
>> -  // leaving this in once KVM gets fixed.
>> -  //
>> -  TimerCtrlReg &= ~ARM_ARCH_TIMER_IMASK;
>>    ArmWriteCntvCtl (TimerCtrlReg);
>>  }
>>  
>> -- 
>> 2.15.1
>>