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Re: [Xen-devel] [PATCH 4/9] xen: arm: turn vtimer traps for cp32/64 and sysreg into #undef

Hi Ian,

On 14/01/15 16:33, Ian Campbell wrote:
> On Thu, 2014-09-11 at 09:43 +0100, Ian Campbell wrote:
>> On Wed, 2014-09-10 at 11:54 -0700, Julien Grall wrote:
>>> Hi Ian,
>>> On 10/09/14 02:46, Ian Campbell wrote:
>>>> On Tue, 2014-09-09 at 16:31 -0700, Julien Grall wrote:
>>>>> Hi Ian,
>>>>> On 09/09/14 09:23, Ian Campbell wrote:
>>>>>> We have allowed EL1 to access these registers directly for some time
>>>>>> (at least since 4.3.0). They were only ever trapped to support very
>>>>>> early models which had a buggy hypervisor timer, requiring us to use
>>>>>> the phys timer for Xen itself.
>>>>>> In the interests of minimising the patch for the security update just
>>>>>> remove the call to vtimer_emulate and inject an #undef exception. In
>>>>>> practice we will never see any of these traps.
>>>>> I disagree with the commit message, a guest may use the physical timer
>>>>> rather than the virtual timer. It's the case when a guest doesn't have
>>>>> the necessary code to use the virtual timer.
> I was just about to dive back into this and was thinking: Is a guest
> which uses the phys timer something we actually wish to support? We
> already require the guest to paravirtualise other aspects of its life,
> and requiring vtimer doesn't seem to step outside that boundary.

Currently if a developer wants to support his OS on Xen, he only has to
add PV drivers (assuming DT support is there). It doesn't have to modify
the core of the OS.

This will be the case with requiring the vtimer. Futhermore, it can be
tricky to implement it on some OS. For instance, the OS may decide to
expose the timer to the user space. Any change would mean recompiler all
the user space for running on Xen.

> Supporting the ptimer is going to take some effort as well as the
> existing code+overhead in Xen.

We already support the physical timer. May I ask, what kind of effort
are necessary?

> Do we know of any existing supported OSes which use the phys timer?

AFAIK no. Even though FreeBSD, while it's not supported, the physical
timer is used by default.


Julien Grall

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