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

[Julien Grall <julien.grall@linaro.org>]

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 think you've misunderstood. The guest is allowed direct access to the
> physical timer ever since we removed the workaround for the buggy
> hypervisor timer on the models. Hence we are never trapping these
> registers anyway. Probably I should go further here and actually remove
> all the phys timer emulation support from vtimer.c.

Are you sure? In init_interrupt_timer (xen/arch/arm/timer.c) we disable 
the access to the physical timer to the guest. See 
WRITE_SYSREG32(CNTHCTL_PA, CNTHCTL_EL2).

Hence, I don't see any save/restore for the physical timer in 
xen/arch/arm/vtimer.c. I only see them for the virtual timer.

Regards,

-- 
Julien Grall