Re: [Xen-devel] [PATCH 1/4] xen/arm: gic: Ensure we have an ISB between ack and do_IRQ()

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 22 Nov 2018, Julien Grall wrote:
> > > If you are worried about performance, then I would recommend to try the
> > > new vGIC and see whether it improves.
> > You know, we are based on XEN 4.10. Initially, when a customer said about
> > their dissatisfaction about performance drop in benchmark due to XEN
> > existence, I tried 4.12-unstable, both an old and a new VGIC. So performance
> > with 4.12-unstable with the old VGIC was worse than 4.10, and the new VGIC
> > made things even much worse. I can't remember the exact numbers or
> > proportions, but that was the reason we do not offer upgrading XEN yet.
> 
> I can't comment without any numbers here. Bear in mind that we fixed bugs in
> Xen 4.12 (including spectre/meltdown and missing barriers) that wasn't
> backported to Xen 4.10. It is entirely possible that it introduced slowness
> but it also ensure the code is behaving correctly.
> 
> Anyway, if there are performance regression we should investigate them and
> discuss how we can address/limit them. Similarly for the new vGIC, if you
> think it is too slow, then we need to know why before we get rid of the old
> vGIC.

Well said! We care about interrupt performance very much and we
definitely need to address any regressions with either the old or the
new driver. But to do that, we need reliable numbers and to figure out
exactly what the problem is so that we can fix it.

I sent the way I used to measure performance in a separate email.

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