Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable pseudo lock

[Reinette Chatre]

Hi Dave,

On 4/11/2024 7:17 AM, Dave Martin wrote:
> On Mon, Apr 08, 2024 at 08:24:12PM -0700, Reinette Chatre wrote:
>> Hi James,
>>
>> On 3/21/2024 9:50 AM, James Morse wrote:
>>> Pseudo-lock relies on knowledge of the micro-architecture to disable
>>> prefetchers etc.
>>>
>>> On arm64 these controls are typically secure only, meaning linux can't
>>> access them. Arm's cache-lockdown feature works in a very different
>>> way. Resctrl's pseudo-lock isn't going to be used on arm64 platforms.
>>>
>>> Add a Kconfig symbol that can be selected by the architecture. This
>>> enables or disables building of the psuedo_lock.c file, and replaces
>>
>> pseudo_lock.c
> 
> Noted.
> 
>>> the functions with stubs. An additional IS_ENABLED() check is needed
>>> in rdtgroup_mode_write() so that attempting to enable pseudo-lock
>>> reports an "Unknown or unsupported mode" to user-space.
>>>
>>
>> I am missing something here. It is not obvious to me why the IS_ENABLED()
>> check is needed. Wouldn't rdtgroup_locksetup_enter()
>> return -EOPNOTSUPP if CONFIG_RESCTRL_FS_PSEUDO_LOCK is not enabled?
>>
>> Reinette
>>
> 
> Hmm, if I've understood all this correctly, then it looks like the
> existing code in rdtgroup_mode_write() relies on the dispatched
> function (rdtgroup_locksetup_enter() etc.) to do an appropriate
> rdt_last_cmd_puts() on failure.  If no function is called at all and
> the requested mode change is not a no-op or otherwise trivially
> successful, then it looks like we're supposed to fall into the else
> clause.
> 
> I'd guess James' intent here was to use the fallback else {} to write
> a suitable status string, while keeping the stub functions as trivial
> as possible.
> 
> Just taking the IS_ENABLED() away would result in error return from the
> write(), but no suitable last_cmd_status string.
> 
> For consistency with the existing x86 implementation, I wonder whether
> we should put a suitable rdt_last_cmd_puts() in the stub for
> rdtgroup_locksetup_enter().
> 
> There might be other ways to refactor or simplify this, though.
> 
> Thoughts?

Thank you for digging into this. It was not obvious to me that
the changelog referred to the last_cmd_status string. I do
not think this warrants making the stubs more complicated.

Reinette