Re: [Xen-devel] [PATCH] arm/vm_event: get/set registers

[Julien Grall <julien.grall@xxxxxxx>]

On 28/07/2016 22:33, Tamas K Lengyel wrote:
> On Jul 28, 2016 15:25, "Julien Grall" <julien.grall@xxxxxxx
> <mailto:julien.grall@xxxxxxx>> wrote:
> > On 28/07/2016 22:05, Tamas K Lengyel wrote:
> > > On Thu, Jul 28, 2016 at 3:01 PM, Julien Grall <julien.grall@xxxxxxx
> <mailto:julien.grall@xxxxxxx>> wrote:
> > > That's not how we do it with vm_event. Even on x86 we only selectively
> > > set registers using the VM_EVENT_FLAG_SET_REGISTERS flag (albeit it
> > > not being documented in the header). As for "not exposing them" it's a
> > > waste to declare separate structures for getting and setting. I'll
> > > change my mind about that if Razvan is on the side that we should
> > > start doing that, but I don't think that's the case at the moment.
> >
> > Is there any rationale to only set a subset of the information you
> retrieved?
> >
> I just did a testrun with setting every register through this method to
> 0 other then pc and it resulted in hypervisor crash. Not sure if it's
> just my setup or not though so I'm still poking at it. However, I don't
> really see a usecase where setting ttbr regs to be required either via
> the fast method so it simply may not worth digging into it more at this
> time.
To confirm, do you mean setting CPSR, TTBR0_EL1, TTBR1_EL1 to 0?

TTBR*_EL1 are safe to set to any values (they are directly accessible by 
the guest anyway). However this is not the case of CPSR. From my 
understanding of the ARM ARM (B1-1150 in ARM DDI 0406C.b) is writing 0 
to M[4:0] will lead to an unpredictable behavior (that could cause an 
hypervisor trap).
Can you copy/paste the hypervisor crash log?

Thank you,

--
Julien Grall

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