[dpdk-dev] [PATCH v4 02/10] eal: add power management intrinsics

[Burakov, Anatoly]

On 08-Oct-20 9:44 AM, Jerin Jacob wrote:
> On Thu, Oct 8, 2020 at 2:04 PM Thomas Monjalon <thomas at monjalon.net> wrote:
>>
>>> Add two new power management intrinsics, and provide an implementation
>>> in eal/x86 based on UMONITOR/UMWAIT instructions. The instructions
>>> are implemented as raw byte opcodes because there is not yet widespread
>>> compiler support for these instructions.
>>>
>>> The power management instructions provide an architecture-specific
>>> function to either wait until a specified TSC timestamp is reached, or
>>> optionally wait until either a TSC timestamp is reached or a memory
>>> location is written to. The monitor function also provides an optional
>>> comparison, to avoid sleeping when the expected write has already
>>> happened, and no more writes are expected.
>>>
>>> For more details, Please reference Intel SDM Volume 2.
>>
>> I really would like to see feedbacks from other arch maintainers.
>> Unfortunately they were not Cc'ed.
> 
> Shared the feedback from the arm64 perspective here. Yet to get a reply on this.
> http://mails.dpdk.org/archives/dev/2020-September/181646.html
> 
>> Also please mark the new functions as experimental.
>>
>>

Hi Jerin,

 > IMO, We must introduce some arch feature-capability _get_ scheme to tell
 > the consumer of this API is only supported on x86. Probably as 
functions[1]
 > or macro flags scheme and have a stub for the other architectures as the
 > API marked as generic ie rte_power_* not rte_x86_..
 >
 > This will help the consumer to create workers based on the 
instruction features
 > which can NOT be abstracted as a generic feature across the 
architectures.

I'm not entirely sure what you mean by that.

I mean, yes, we should have added stubs for other architectures, and we 
will add those in future revisions, but what does your proposed runtime 
check accomplish that cannot currently be done with CPUID flags?

If you look at patch 1 [1], we added CPUID flags that the user can 
check, and in fact this is precisely what we do in patch 4 [2] before 
enabling the UMWAIT path. We could perhaps document this better and 
outline the dependency on the WAITPKG CPUID flag more explicitly, but 
otherwise i don't see how what you're proposing isn't already possible 
to do.

[1] http://patches.dpdk.org/patch/79539/
[2] http://patches.dpdk.org/patch/79540/ , function 
rte_power_pmd_mgmt_queue_enable()

-- 
Thanks,
Anatoly