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Date: Wed, 17 Apr 2024 15:42:28 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: Reinette Chatre <reinette.chatre@intel.com>
Cc: James Morse <james.morse@arm.com>, x86@kernel.org,
	linux-kernel@vger.kernel.org, Fenghua Yu <fenghua.yu@intel.com>,
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	Rex Nie <rex.nie@jaguarmicro.com>
Subject: Re: [PATCH v1 20/31] x86/resctrl: Allow an architecture to disable
 pseudo lock
Message-ID: <Zh/f1J0sWtK8MswG@e133380.arm.com>
References: <20240321165106.31602-1-james.morse@arm.com>
 <20240321165106.31602-21-james.morse@arm.com>
 <6d563361-87c0-474d-812c-b33e79eacf3a@intel.com>
 <Zhfw/uEZaYk9492j@e133380.arm.com>
 <172f4615-8943-448c-ac68-152d5bcffa76@intel.com>
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In-Reply-To: <172f4615-8943-448c-ac68-152d5bcffa76@intel.com>

Hi Reinette,

On Thu, Apr 11, 2024 at 10:40:03AM -0700, Reinette Chatre wrote:
> 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
> 

OK, I'll leave this as-is for now.

Cheers
---Dave