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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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

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