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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 07/29/16 00:25, Julien Grall 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>
>> 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?

The perennial speed optimization, but mainly that setting everything can
have side-effects (on x86 I remember that at the time I wrote the
initial patch this had something to do with the control registers - if
you'd like I can try to follow the code again and try to remember what
the exact issue was).

My main use-case at the time was to simply set EIP (I needed to be able
to skip the current instruction if it happened to be deemed to be
malicious by the introspection engine). I believe it has been assumed at
the time that setting the GPRS is enough, and that can be extended in
the future by interested parties.


Thanks,
Razvan

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