Re: [PATCH 5/6] hw/ppc: Always store the decrementer value

[Cédric Le Goater]

On 7/30/23 11:40, Nicholas Piggin wrote:
> On Thu Jul 27, 2023 at 10:26 PM AEST, Cédric Le Goater wrote:
> > Hello Nick,
> >
> > On 7/26/23 20:22, Nicholas Piggin wrote:
> > > When writing a value to the decrementer that raises an exception, the
> > > irq is raised, but the value is not stored so the store doesn't appear
> > > to have changed the register when it is read again.
> > >
> > > Always store the write value to the register.
> >
> > This change has a serious performance impact when a guest is run under
> > PowerNV. Could you please take a look ?
>
> Yeah, the decrementer load doesn't sign-extend the value correctly as
> it should for the large-decrementer option. It makes skiboot detect
> the decrementer size as 64 bits instead of 56, and things go bad from
> there. KVM seems more affected because it's saving and restoring DEC
> frequently.
>
> The fix seems simple but considering the compounding series of bugs
> and issues coming up with this, I think it will be better to defer
> the decrementer work until 8.2 unfortunately.

Yes. QEMU 8.1 has already a lot, fixes, tests and models [1].

> > PS: We should really introduce avocado tests for nested.
>
> Yeah agreed. Both for pseries and powernv, ideally.

The same disk image could be used for the 3 HV implementations. This would
be a nice addition to Harsh's series [2]

Thanks,

C.

[1] https://wiki.qemu.org/ChangeLog/8.1#PowerPC
[2] https://patchwork.ozlabs.org/project/qemu-ppc/list/?series=364386