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

[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?

Thanks,
-Christoffer