Re: [PATCH 4/4] powerpc/64s: Use POWER10 stsync barrier for wmb()

[Michael Ellerman <mpe@ellerman.id.au>]

Michael Ellerman <mpe@ellerman.id.au> writes:
> Michael Ellerman <mpe@ellerman.id.au> writes:
>> "Nicholas Piggin" <npiggin@gmail.com> writes:
>>> On Wed Jun 14, 2023 at 3:56 PM AEST, Michael Ellerman wrote:
>>>> Michael Ellerman <mpe@ellerman.id.au> writes:
>>>> > Nicholas Piggin <npiggin@gmail.com> writes:
>>>> >> The most expensive ordering for hwsync to provide is the store-load
>>>> >> barrier, because all prior stores have to be drained to the caches
>>>> >> before subsequent instructions can complete.
>>>> >>
>>>> >> stsync just orders stores which means it can just be a barrer that
>>>> >> goes down the store queue and orders draining, and does not prevent
>>>> >> completion of subsequent instructions. So it should be faster than
>>>> >> hwsync.
>>>> >>
>>>> >> Use stsync for wmb(). Older processors that don't recognise the SC
>>>> >> field should treat this as hwsync.
>>>> >
>>>> > qemu (7.1) emulating ppc64e does not :/
>>>> >
>>>> >   mpic: Setting up MPIC " OpenPIC  " version 1.2 at fe0040000, max 1 CPUs
>>>> >   mpic: ISU size: 256, shift: 8, mask: ff
>>>> >   mpic: Initializing for 256 sources
>>>> >   Oops: Exception in kernel mode, sig: 4 [#1]
>>>> ..
>>>> >
>>>> > I guess just put it behind an #ifdef 64S.
>>>>
>>>> That doesn't work because qemu emulating a G5 also doesn't accept it.
>>>>
>>>> So either we need to get qemu updated and wait a while for that to
>>>> percolate, or do some runtime patching of wmbs in the kernel >_<
>>>
>>> Gah, sorry. QEMU really should be ignoring reserved fields in
>>> instructions :(
>>
>> Yeah, it's an annoying discrepancy vs real hardware and the ISA.
>>
>>> I guess leave it out for now. Should fix QEMU but we probably also need
>>> to do patching so as not to break older QEMUs.
>>
>> I'll plan to take the first 3 patches, they seem OK as-is.
>
> I didn't do that in the end, because patch 2 suffers from the same
                                             ^
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> problem of not working on QEMU.
>
> cheers