Message-ID: <20250318-bf7e13879b2073c610d32bae@orel>
Date: Tue, 18 Mar 2025 16:09:48 +0100
From: Andrew Jones <ajones@...tanamicro.com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, paul.walmsley@...ive.com, palmer@...belt.com, 
	charlie@...osinc.com, cleger@...osinc.com, Anup Patel <apatel@...tanamicro.com>, 
	corbet@....net
Subject: Re: [PATCH v3 7/8] riscv: Add parameter for skipping access speed
 tests

On Tue, Mar 18, 2025 at 03:09:18PM +0100, Alexandre Ghiti wrote:
> On 18/03/2025 14:04, Andrew Jones wrote:
> > On Tue, Mar 18, 2025 at 01:58:10PM +0100, Alexandre Ghiti wrote:
> > > On 18/03/2025 13:45, Andrew Jones wrote:
> > > > On Tue, Mar 18, 2025 at 01:13:18PM +0100, Alexandre Ghiti wrote:
> > > > > On 18/03/2025 09:48, Andrew Jones wrote:
> > > > > > On Mon, Mar 17, 2025 at 03:39:01PM +0100, Alexandre Ghiti wrote:
> > > > > > > Hi Drew,
> > > > > > > 
> > > > > > > On 04/03/2025 13:00, Andrew Jones wrote:
> > > > > > > > Allow skipping scalar and vector unaligned access speed tests. This
> > > > > > > > is useful for testing alternative code paths and to skip the tests in
> > > > > > > > environments where they run too slowly. All CPUs must have the same
> > > > > > > > unaligned access speed.
> > > > > > > I'm not a big fan of the command line parameter, this is not where we should
> > > > > > > push uarch decisions because there could be many other in the future, the
> > > > > > > best solution to me should be in DT/ACPI and since the DT folks, according
> > > > > > > to Palmer, shut down this solution, it remains using an extension.
> > > > > > > 
> > > > > > > I have been reading a bit about unaligned accesses. Zicclsm was described as
> > > > > > > "Even though mandated, misaligned loads and stores might execute extremely
> > > > > > > slowly. Standard software distributions should assume their existence only
> > > > > > > for correctness, not for performance." in rva20/22 but *not* in rva23. So
> > > > > > > what about using this "hole" and consider that a platform that *advertises*
> > > > > > > Zicclsm means its unaligned accesses are fast? After internal discussion, It
> > > > > > > actually does not make sense to advertise Zicclsm if the platform accesses
> > > > > > > are slow right?
> > > > > > This topic pops up every so often, including in yesterday's server
> > > > > > platform TG call. In that call, and, afaict, every other time it has
> > > > > > popped up, the result is to reiterate that ISA extensions never say
> > > > > > anything about performance. So, Zicclsm will never mean fast and we
> > > > > > won't likely be able to add any extension that does.
> > > > > Ok, I should not say "fast". Usually, when an extension is advertised by a
> > > > > platform, we don't question its speed (zicboz, zicbom...etc), we simply use
> > > > > it and it's up to the vendor to benchmark its implementation and act
> > > > > accordingly (i.e. do not set it in the isa string).
> > > > > 
> > > > > 
> > > > > > > arm64 for example considers that armv8 has fast unaligned accesses and can
> > > > > > > then enable HAVE_EFFICIENT_ALIGNED_ACCESS in the kernel, even though some
> > > > > > > uarchs are slow. Distros will very likely use rva23 as baseline so they will
> > > > > > > enable Zicclsm which would allow us to take advantage of this too, without
> > > > > > > this, we lose a lot of perf improvement in the kernel, see
> > > > > > > https://lore.kernel.org/lkml/20231225044207.3821-1-jszhang@kernel.org/.
> > > > > > > 
> > > > > > > Or we could have a new named feature for this, even though it's weird to
> > > > > > > have a named feature which would basically  mean "Zicclsm is fast". We don't
> > > > > > > have, for example, a named feature to say "Zicboz is fast" but given the
> > > > > > > vague wording in the profile spec, maybe we can ask for one in that case?
> > > > > > > 
> > > > > > > Sorry for the late review and for triggering this debate...
> > > > > > No problem, let's try to pick the best option. I'll try listing all the
> > > > > > options and there pros/cons.
> > > > > > 
> > > > > > 1. Leave as is, which is to always probe
> > > > > >       pro: Nothing to do
> > > > > >       con: Not ideal in all environments
> > > > > > 
> > > > > > 2. New DT/ACPI description
> > > > > >       pro: Describing whether or not misaligned accesses are implemented in
> > > > > >            HW (which presumably means fast) is something that should be done
> > > > > > 	in HW descriptions
> > > > > >       con: We'll need to live with probing until we can get the descriptions
> > > > > >            defined, which may be never if there's too much opposition
> > > > > > 
> > > > > > 3. Command line
> > > > > >       pro: Easy and serves its purpose, which is to skip probing in the
> > > > > >            environments where probing is not desired
> > > > > >       con: Yet another command line option (which we may want to deprecate
> > > > > >            someday)
> > > > > > 
> > > > > > 4. New ISA extension
> > > > > >       pro: Easy to add to HW descriptions
> > > > > >       con: Not likely to get it through ratification
> > > > > > 
> > > > > > 5. New SBI FWFT feature
> > > > > >       pro: Probably easier to get through ratification than an ISA extension
> > > > > >       con: Instead of probing, kernel would have to ask SBI -- would that
> > > > > >            even be faster? Will all the environments that want to skip
> > > > > > 	probing even have a complete SBI?
> > > > > > 
> > > > > > 6. ??
> > > > > So what about:
> > > > > 
> > > > > 7. New enum value describing the performance as "FORCED" or "HW" (or
> > > > > anything better)
> > > > >       pro: We only use the existing Zicclsm
> > > > >       con: It's not clear that the accesses are fast but it basically says to
> > > > > SW "don't think too much, I'm telling you that you can use it", up to us to
> > > > > describe this correctly for users to understand.
> > > > But Zicclsm doesn't mean misaligned accesses are in HW, it just means
> > > > they're not going to explode.
> > > 
> > > They never explode since if they are not supported by the HW, we rely on
> > > S-mode emulation already.
> > Exactly. Zicclsm is just a new name for that behavior. Profiles try to
> > name every behavior, even the ones we take for granted. Unfortunately,
> > like in the case of Zicclsm, we don't necessarily gain anything from
> > the new name. In this case, we don't gain a way to avoid probing.
> 
> 
> I understand your point but given the misaligned traps exist, I can't find
> another meaning to Zicclsm than "I'm telling you to use it". Zicclsm can't
> be used to describe an OS behaviour (ie the emulation of misaligned
> accesses).
> 
> I'm also insisting because we need a compile-time hint which allows us to
> enable HAVE_EFFICIENT_UNALIGNED_ACCESS in the kernel and Zicclsm is great
> since it is required in RVA23. if that's not Zicclsm, that must be another
> named feature/extension.
> 
> What do you suggest to make progress here?
>

I guess you mean besides listing five options and posting patches for two
of them :-)  We can't force semantics onto Zicclsm and I doubt we'll get
agreement to make another extension with the semantics we want. So (4)
is out. I agree with Clement that (5) isn't good. That leaves (2). I
guess we should start by trying to understand what issues there were/are
with it.

Thanks,
drew

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