Re: [RFC v4 00/10] Provide the EL1 physical timer to the VM

[Jintack Lim]

On Fri, Feb 3, 2017 at 11:14 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
> On 03/02/17 15:19, 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 only on arm64. 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.
>>
>> v3 => v4:
>>  - Fix a bug that prevents a VM from booting on 32-bit architecture
>>  - Clarify that the emulated physical timer is only supported on arm64
>>    in the cover letter
>
> Hi Jintack,
>
> I've now applied this to queue, and will push it out later today.

Thanks, Marc.

>
> Out of curiosity, is there any reason why this is arm64 only?

It was simply because I didn't have a convenient 32bit architecture
develop environment at hand and didn't spend time to set it up myself
:(
(As specified in the nesting RFC patch series cover letter, the
nesting patches are compiled, but not tested on 32-bit architecture
yet.)
I guess it's time to set it up.

> As far as
> I can tell, we're only missing the cp15 handling (both for arm and in
> the 32bit handling in arm64).

I think so, too. I can't promise when, but I'll try to add those once
I set the develop environment.

>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny...
>