Re: [RFC v3 00/10] Provide the EL1 physical timer to the VM

[Jintack Lim]

On Thu, Feb 2, 2017 at 7:31 AM, Christoffer Dall <cdall@xxxxxxxxxx> wrote:
> Hi Jintack,
>
> On Wed, Feb 01, 2017 at 12:43:00PM -0500, Jintack Lim wrote:
>> The ARM architecture defines the EL1 physical timer and the virtual timer,
>> and it is reasonable for an OS to expect to be able to access both.
>> However, the current KVM implementation does not provide the EL1 physical
>> timer to VMs but terminates VMs on access to the timer.
>>
>> This patch series enables VMs to use the EL1 physical timer through
>> trap-and-emulate.  The KVM host emulates each EL1 physical timer register
>> access and sets up the background timer accordingly.  When the background
>> timer expires, the KVM host injects EL1 physical timer interrupts to the
>> VM.  Alternatively, it's also possible to allow VMs to access the EL1
>> physical timer without trapping.  However, this requires somehow using the
>> EL2 physical timer for the Linux host while running the VM instead of the
>> EL1 physical timer.  Right now I just implemented trap-and-emulate because
>> this was straightforward to do, and I leave it to future work to determine
>> if transferring the EL1 physical timer state to the EL2 timer provides any
>> performance benefit.
>>
>> This feature will be useful for any OS that wishes to access the EL1
>> physical timer. Nested virtualization is one of those use cases. A nested
>> hypervisor running inside a VM would think it has full access to the
>> hardware and naturally tries to use the EL1 physical timer as Linux would
>> do. Other nested hypervisors may try to use the EL2 physical timer as Xen
>> would do, but supporting the EL2 physical timer to the VM is out of scope
>> of this patch series. This patch series will make it easy to add the EL2
>> timer support in the future, though.
>>
>> Note that Linux VMs booting in EL1 will be unaffected by this patch series
>> and will continue to use only the virtual timer and this patch series will
>> therefore not introduce any performance degredation as a result of
>> trap-and-emulate.
>>
>> v2 => v3:
>>  - Rebase on kvmarm/queue
>>  - Take kvm->lock to synchronize cntvoff across all vtimers
>>  - Remove unnecessary function parameters
>>  - Add comments
>
> I just gave v3 a test run on my TC2 (32-bit platform) and my guest
> quickly locks up trying to run cyclictest or when booting the machine it
> stalls with RCU timeouts.

Ok. It's my fault not to specify that the emulated physical timer is
supported/tested on arm64.
On 32-bit platform, it is supposed to show the same behavior as
before, but I haven't tested.
Were you using the physical timer or the virtual timer for the guest?

>
> Could you have a look?

Sure, I'll have a look. I don't have access to my Cubietruck today,
but I can work on that tomorrow.

>
> Thanks,
> -Christoffer
>