[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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.


Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.