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Re: [Xen-devel] [PATCH v2 2/5] vm_event: Implement ARM SMC events

On Wed, May 4, 2016 at 8:03 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
> On 04/05/2016 14:30, Razvan Cojocaru wrote:
>> On 05/04/2016 04:26 PM, Julien Grall wrote:
>>> I may misunderstand some parts of the vm event subsystem. To get/set the
>>> full set of registers, the user will have to use the
>>> DOMCTL_{set,get}vcpucontext, correct? So why does Xen expose a part of
>>> the vCPU context through the vm_event?
>> Because DOMCTL_{set,get}vcpucontext is expensive, and a serious
>> introspection application will receive hundreds or thousands of events
>> per second.
>> Having what's most commonly needed being sent with the vm_event
>> eliminates the need for extra hypercalls and can make the difference
>> between a responsive and an unusable userspace introspection application.
> Thank you for the explanation. So in this case, we should also make x16
> (AArch64) and r12 (AArch32) available as they will be used for hypercall.

That's fine by me. At the moment the only registers I have definite
use for is pc and ttbr0/1, the others are up for grabs.


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