[PATCH v2] arm/arm64: KVM: Properly account for guest CPU time

[Christoffer Dall]

On Mon, Jun 01, 2015 at 03:37:32PM +0200, Christian Borntraeger wrote:
> Am 01.06.2015 um 15:35 schrieb Christoffer Dall:
> > On Mon, Jun 01, 2015 at 11:21:19AM +0200, Christian Borntraeger wrote:
> >> Am 01.06.2015 um 11:08 schrieb Christoffer Dall:
> >>
> >>>>>
> >>>>> Second, looking at the ppc and mips code, they seem to also call
> >>>>> kvm_guest_exit() before enabling interrupts, so I don't understand how
> >>>>> guest CPU time accounting works on those architectures.
> >>>>
> >>>> Not an expert here, but I assume mips has the same logic as arm so if your
> >>>> patch is right for arm its probably also for mips.
> >>>>
> >>>> powerpc looks similar to what s390 does (not using the tick, instead it uses
> >>>> a hw-timer) so this should be fine.
> >>>>
> >>> I wonder if we can simply enable HAVE_VIRT_CPU_ACCOUNTING_GEN and get
> >>> this for free which would avoid the need for this patch?
> >>
> >> Asssuming that HAVE_VIRT_CPU_ACCOUNTING_GEN behaves similar to 
> >> HAVE_VIRT_CPU_ACCOUNTING on s390/power in respect to not rely on ticks
> >> - yes it might work out. Can you give it a try?
> >>
> > Adding HAVE_VIRT_CPU_ACCOUNTING_GEN to arch/arm64/Kconfig works, but has
> > no effect unless you also enable CONFIG_NO_HZ_FULL, so that hardly feels
> > like a fix since it would be a shame to force users to use this config
> > option to report CPU usage correctly.
> > 
> > I'm not entirely sure what the history and meaning behind these configs
> > are, so maybe there is an entirely different rework needed here.  It
> > seems logical that you could simply sample the counter at entry/exit of
> > the guest, but if there is nowhere to store this data without
> > NO_HZ_FULL+VIRT_CPU_ACCOUNTING_GEN then I guess that would be why?
> 
> Given Paolos response that irq_disable/enable is faster than save/restore
> at least on x86 your v2 patch might actually be the right thing to do.
> 
Thanks, I think so too, but we should enable
HAVE_VIRT_CPU_ACCOUNTING_GEN for arm64 as well, assuming there are no
mysterious side affects of doing so.

-Christoffer