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

[Marc Zyngier <marc.zyngier@arm.com>]

On 03/02/17 16:51, Jintack Lim wrote:
> On Fri, Feb 3, 2017 at 11:14 AM, Marc Zyngier <marc.zyngier@arm.com> 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.

That's fine, we can add these later (and maybe I'll just do it, since it
is pretty trivial).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...