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

[Nicholas Piggin]

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.

Thanks,
Nick

> Thanks,
>
> C.
>
> PS: We should really introduce avocado tests for nested.

Yeah agreed. Both for pseries and powernv, ideally.

Thanks,
Nick