Re: [PATCH 11/11] powerpc/smp: Add a doorbell=off kernel parameter

["Cédric Le Goater" <clg@kaod.org>]

On 11/18/21 10:24, Michael Ellerman wrote:
> Cédric Le Goater <clg@kaod.org> writes:
>> On 11/11/21 11:41, Michael Ellerman wrote:
>>> Cédric Le Goater <clg@kaod.org> writes:
>>>> On processors with a XIVE interrupt controller (POWER9 and above), the
>>>> kernel can use either doorbells or XIVE to generate CPU IPIs. Sending
>>>> doorbell is generally preferred to using the XIVE IC because it is
>>>> faster. There are cases where we want to avoid doorbells and use XIVE
>>>> only, for debug or performance. Only useful on POWER9 and above.
>>>
>>> How much do we want this?
>>
>> Yes. Thanks for asking. It is a recent need.
>>
>> Here is some background I should have added in the first place. May be
>> for a v2.
>>
>> We have different ways of doing IPIs on POWER9 and above processors,
>> depending on the platform and the underlying hypervisor.
>>
>> - PowerNV uses global doorbells
>>
>> - pSeries/KVM uses XIVE only because local doorbells are not
>>     efficient, as there are emulated in the KVM hypervisor
>>
>> - pSeries/PowerVM uses XIVE for remote cores and local doorbells for
>>     threads on same core (SMT4 or 8)
>>
>> This recent commit 5b06d1679f2f ("powerpc/pseries: Use doorbells even
>> if XIVE is available") introduced the optimization for PowerVM and
>> commit 107c55005fbd ("powerpc/pseries: Add KVM guest doorbell
>> restrictions") restricted the optimization.
>>
>> We would like a way to turn off the optimization.
> 
> Just for test/debug though?

Yes. For now, this is the main target.

>>> Kernel command line args are a bit of a pain, they tend to be poorly
>>> tested, because someone has to explicitly enable them at boot time,
>>> and then reboot to test the other case.
>>
>> True. The "xive=off" parameter was poorly tested initially.
>>
>>> When would we want to enable this?
>>
>> For bring-up, for debug, for tests. I have been using a similar switch
>> to compare the XIVE interrupt controller performance with doorbells on
>> POWER9 and P0WER10.
>>
>> A new need arises with PowerVM, some configurations will behave as KVM
>> (local doorbell are unsupported) and the doorbell=off parameter is a
>> simple way to handle this case today.
> 
> That's the first I've heard of that, what PowerVM configurations have
> non-working doorbells?

It's not released yet.

>>> Can we make the kernel smarter about when to use doorbells and make
>>> it automated?
>>
>> I don't think we want to probe all IPI methods to detect how well
>> local doorbells are supported on the platform. Do we ?
> 
> We don't *want to*, but sounds like we might have to if they are not
> working in some configurations as you mentioned above.

Yeah. This is too early. I will keep that patch for internal use
until we have clarified.

Thanks,

C.