Re: [Xen-devel] [RFC 1/6] xen/arm: Re-enable interrupt later in the trap path

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrii,

On 02/08/2019 13:24, Andrii Anisov wrote:
> On 02.08.19 12:03, Julien Grall wrote:
> > A fair amount of leave_hypervisor_tail() deal with the guest itself (i.e vGIC, 
> > P2M...)
> All that stuff is what hypervisor does for the guest. And does behind the 
> guest's back.
Please define "guest's back".

Technically a guest accessing an IO does not know that the access will be 
emulated. So this should also take into account as "guest's back".
An hypercall is initiated by the guest directly, so I agree this is not done on 
guest's back.
Some of the work done in leave_hypervisor_tail() is an extension of IO 
emulation. They are not done directly in the IO emulation because they may take 
a long time and get preemption. So I don't see any difference with "IO emulation".
Regarding the vGIC. This is a bit more a grey area. While it is an overhead of 
virtualization, this is indirectly initiated by the guest. Indeed, you would 
only configure the vGIC if you receive an interrupt generated by one of the 
device assigned.
> > , so I think they should be accounted to the guest time.
> This point is arguable. That's why we have a discussion here to agree on the 
> time accounting approach, what will directly affect scheduling accuracy.
Note the "I think" in my answer. So this is my opinion and your input is 
expected.

Cheers,

--
Julien Grall

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