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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

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

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

I suppose we should consider access to the counter separately from
access to the registers which allow event generation -- I could
plausibly be convinced that a guest should be able to read both phys and
virt timers (for e.g. stolen time type reasons), but only configure virt
events.

Ian.

> > >
> > > 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).
> 
> Hrm, I mistakenly thought that was enabling them, but we do indeed need
> to set a second bit there, this only allows access to the counter, not
> the control registers. I'll take another look.
> 
> > 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,
> > 
> 
> 
> 
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