[PATCH v5 1/4] RISC-V: Add Svade and Svadu Extensions Support

[Andrew Jones]

On Fri, Jun 21, 2024 at 12:00:32PM GMT, Conor Dooley wrote:
> On Fri, Jun 21, 2024 at 12:42:32PM +0200, Andrew Jones wrote:
> > On Fri, Jun 21, 2024 at 11:24:19AM GMT, Conor Dooley wrote:
> > > On Fri, Jun 21, 2024 at 10:43:58AM +0200, Andrew Jones wrote:
...
> > > > It's hard to guess what is, or will be, more likely to be the correct
> > > > choice of call between the _unlikely and _likely variants. But, while we
> > > > assume svade is most prevalent right now, it's actually quite unlikely
> > > > that 'svade' will be in the DT, since DTs haven't been putting it there
> > > > yet. Anyway, it doesn't really matter much and maybe the _unlikely vs.
> > > > _likely variants are better for documenting expectations than for
> > > > performance.
> > > 
> > > binding hat off, and kernel hat on, what do we actually do if there's
> > > neither Svadu or Svade in the firmware's description of the hardware?
> > > Do we just arbitrarily turn on Svade, like we already do for some
> > > extensions:
> > > 	/*
> > > 	 * These ones were as they were part of the base ISA when the
> > > 	 * port & dt-bindings were upstreamed, and so can be set
> > > 	 * unconditionally where `i` is in riscv,isa on DT systems.
> > > 	 */
> > > 	if (acpi_disabled) {
> > > 		set_bit(RISCV_ISA_EXT_ZICSR, isainfo->isa);
> > > 		set_bit(RISCV_ISA_EXT_ZIFENCEI, isainfo->isa);
> > > 		set_bit(RISCV_ISA_EXT_ZICNTR, isainfo->isa);
> > > 		set_bit(RISCV_ISA_EXT_ZIHPM, isainfo->isa);
> > > 	}
> > >
> > 
> > Yes, I think that's reasonable, assuming we do it in the final "pass",
> > where we're sure svadu isn't present.
> 
> I haven't thought about specifically when to do it, but does it need to
> be in the final pass? If we were to, on each CPU, enable it if Svadu
> isn't there, we'd either end up with a system that I suspect we're not
> going to be supporting or the correct result. Or am I misunderstanding,
> and it will be valid to have a subset of CPUs that have Svadu enabled
> from the bootloader?
> 
> Note that it would not be problematic to have 3 CPUs with Svade + Svadu
> and a 4th with only Svade in the DT because we would just not use the
> FWFT mechanism to enable Svadu. It's just the Svadu in isolation case
> that I'm asking about.

I wasn't thinking about the potential of mixmatched A/D udpating. I'm
pretty sure this will be one of those things that is all or none. I
was more concerned with getting the result right and I had just been
too lazy to double check that the block of code you pointed out is
in the right place to be sure there's no svadu. Now that I look, I
believe it is.

> 
> > Doing this is a good idea since
> > we'll be able to simplify conditions, as we can just use 'if (svade)'
> > since !svade would imply svadu. With FWFT and both, we'll want to remove
> > svade from the isa bitmap when enabling svadu.
> 
> Right I would like to move the various extension stuff in this
> direction, where they have a bit more intelligence to them, and don't
> just reflect the state in DT/ACPI directly.
> I've got some patches in mind once Clement's Zca etc patchset
> is merged, think I posted one or two as replies to conversations on
> the list already. An example would be disabling the vector crypto
> extensions if we've had to disable vector, or as you suggest here,
> dropping Svade if we have turned on Svadu using FWFT. I think that makes
> the APIs more understandable to developers and more useful than they are
> at the moment, where to use vector crypto you also need to check vector
> itself for the code to be correct. If I call
> riscv_isa_extension_available(), and it returns true, the extension
> should be usable IMO.

Sounds good to me.

Thanks,
drew