Re: [Xen-devel] [PATCH V2] firmware: Change level-triggered GPE event to a edge one for qemu-xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.12.13 at 18:11, Anthony PERARD <anthony.perard@xxxxxxxxxx> wrote:
> On Fri, Dec 13, 2013 at 11:51:19AM +0000, Jan Beulich wrote:
>> >>> On 12.12.13 at 16:30, Anthony PERARD <anthony.perard@xxxxxxxxxx> wrote:
>> > Change in V2:
>> >   - better patch comment:
>> >     patch those not fix race, but reduce the window
>> >     include patch description of the quoted commit
>> 
>> Thanks - quite a bit better to understand.
>> 
>> >   - change also apply to pci hotplug.
>> 
>> The one thing I'm still missing for both changes is a brief word on
>> why qemu-xen-traditional doesn't want/need this and - as iirc
>> you said - if that one manages to get this implemented without a
>> similar race, why upstream qemu can't do things in a similar way.
> 
> In qemu-trad, instead of sending an SCI interrupt for every new vcpu, we
> loop through every xenstore key that represent vcpus availability and
> only send an SCI only when the loop is over. And it looks like one `xl
> vcpu-set` provoc only one loop.
> 
> But adding a vcpu in qemu-xen is done via a QMP command, "cpu-add id=X".
> qemu-xen have no way to know if there will be a next cpu-add command, so
> we can not apply the same thing.
> 
> For why qemu-xen-traditional doesn't not need this:
>   - a single `xl vcpu-set` can not trigger the race
> 
> For why qemu-xen-traditional doesn't not want this:
>   - avoid unnecessary change, especially in the ACPI table
> 
> About the PCI hotplug change and qemu-xen-traditional, I actually don't
> know what would be the effect.
> 
> Why qemu-xen-traditional would like this change for vcpu only:
>   - we might hit the race window via several consecutive `xl vcpu-set`
>     calls, or via direct change of xenstore entries.
> but after few try, I could not manage to have a missing cpu hotplug
> event.
> 
> 
> So should I just add in the patch comment something like:
> "Since some measure have been put in place in qemu-xen-traditional
> against this race, we don't change the ACPI table for it."
> ?

Yes please.

Jan


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