Re: [PATCH v2 1/3] eal: add x86 cpuid support for monitorx

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 4/14/2023 8:05 AM, David Marchand wrote:
> On Thu, Apr 13, 2023 at 7:51 PM Tummala, Sivaprasad
> <Sivaprasad.Tummala@amd.com> wrote:
>>
>> [AMD Official Use Only - General]
>>
>> Hi David,
>>
>>> -----Original Message-----
>>> From: David Marchand <david.marchand@redhat.com>
>>> Sent: Thursday, April 13, 2023 5:30 PM
>>> To: Tummala, Sivaprasad <Sivaprasad.Tummala@amd.com>
>>> Cc: david.hunt@intel.com; dev@dpdk.org; Thomas Monjalon
>>> <thomas@monjalon.net>; Burakov, Anatoly <anatoly.burakov@intel.com>
>>> Subject: Re: [PATCH v2 1/3] eal: add x86 cpuid support for monitorx
>>>
>>> Caution: This message originated from an External Source. Use proper caution
>>> when opening attachments, clicking links, or responding.
>>>
>>>
>>> On Thu, Apr 13, 2023 at 1:54 PM Sivaprasad Tummala
>>> <sivaprasad.tummala@amd.com> wrote:
>>>>
>>>> Add a new CPUID flag to indicate support for monitorx instruction on
>>>> AMD Epyc processors.
>>>>
>>>> Signed-off-by: Sivaprasad Tummala <sivaprasad.tummala@amd.com>
>>>> ---
>>>>  lib/eal/include/generic/rte_cpuflags.h | 2 ++
>>>>  lib/eal/x86/include/rte_cpuflags.h     | 1 +
>>>>  lib/eal/x86/rte_cpuflags.c             | 3 +++
>>>>  3 files changed, 6 insertions(+)
>>>>
>>>> diff --git a/lib/eal/include/generic/rte_cpuflags.h
>>>> b/lib/eal/include/generic/rte_cpuflags.h
>>>> index d35551e931..db653a8dd7 100644
>>>> --- a/lib/eal/include/generic/rte_cpuflags.h
>>>> +++ b/lib/eal/include/generic/rte_cpuflags.h
>>>> @@ -26,6 +26,8 @@ struct rte_cpu_intrinsics {
>>>>         /**< indicates support for rte_power_pause function */
>>>>         uint32_t power_monitor_multi : 1;
>>>>         /**< indicates support for rte_power_monitor_multi function */
>>>> +       uint32_t amd_power_monitorx : 1;
>>>> +       /**< indicates amd support for rte_power_monitor function */
>>>
>>> I did not look at the patch detail, I just stopped at this part.
>>> What makes the AMD monitorx stuff special that it needs to be exposed in the
>>> generic API?
>>
>> Monitorx is different ISA /opcode (0F 01 FA) as compared to UMonitor (0F 01 C8). This need to be distinguished
>> on specific x86 platforms. Hence in the current power intrinsics, for x86 we require a new flag to
>> distinguish MonitorX and UMonitor and invoke the appropriate x86 ISA in the datapath.
> 
> Requiring a new x86 cpuflag to identify this ISA presence is ok.
> 
> 
> But here, I am talking about the generic power instrinsic API.
> Let me phrase my comment differently...
> 
> As described in the API:
>         uint32_t power_monitor : 1;
>         /**< indicates support for rte_power_monitor function */
> 
> Does AMD thing behave completely different from the x86?
> Looking at patch 3, I understand this is not the case.
> 
> So we don't need a "amd" flag in the generic flags.
> The indirection for calling the right ISA should be hidden in
> rte_power_* helpers implemented for x86.
> 
> 


The 'rte_cpu_get_intrinsics_support()' API and "struct
rte_cpu_intrinsics" struct seems intended to get power features in
generic way, agree to keep it generic.

But also there is a need to run architecture specific instructions, so
need to know the architecture within power library, for this what do you
think to check 'MONITORX' support again in 'rte_power_intrinsics_init()'
function?


And most of the 'amd_power_monitorx()' function is duplicate of the
'rte_power_monitor()' API, only difference is the asm calls, what do you
think to extract these calls to function pointers for AMD and Intel, so
that 'rte_power_monitor()' can become a x86 generic function?

As architecture will be known in the 'rte_power_intrinsics_init()', we
can set the function pointers properly for architecture in this init stage.

Only concern is possible performance impact of pointer dereference
instead of direct call, I hope @David Hunt can help us to test the
performance impact of it in Intel platforms if this approach is reasonable.