Re: [PATCH 08/19] riscv: Introduce vendor variants of extension helpers

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Apr 12, 2024 at 09:40:03PM +0100, Conor Dooley wrote:
> On Fri, Apr 12, 2024 at 10:43:02AM -0700, Charlie Jenkins wrote:
> > On Fri, Apr 12, 2024 at 12:49:57PM +0100, Conor Dooley wrote:
> > > On Thu, Apr 11, 2024 at 09:11:14PM -0700, Charlie Jenkins wrote:
> > > > Create vendor variants of the existing extension helpers. If the
> > > > existing functions were instead modified to support vendor extensions, a
> > > > branch based on the ext value being greater than
> > > > RISCV_ISA_VENDOR_EXT_BASE would have to be introduced. This additional
> > > > branch would have an unnecessary performance impact.
> > > > 
> > > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > > 
> > > I've not looked at the "main" patch in the series that adds all of the
> > > probing and structures for representing this info yet beyond a cursory
> > > glance, but it feels like we're duplicating a bunch of infrastructure
> > > here before it is necessary. The IDs are all internal to Linux, so I'd
> > > rather we kept everything in the same structure until we have more than
> > > a handful of vendor extensions. With this patch (and the theadpmu stuff)
> > > we will have three vendor extensions which feels like a drop in the
> > > bucket compared to the standard ones.
> > 
> > It is not duplicating infrastructure. If we merge this into the existing
> > infrastructure, we would be littering if (ext > RISCV_ISA_VENDOR_EXT_BASE)
> > in __riscv_isa_extension_available. This is particularily important
> > exactly because we have so few vendor extensions currently so this check
> > would be irrelevant in the vast majority of cases.
> 
> That's only because of your implementation. The existing vendor extension
> works fine without this littering. That's another thing actually, you
> forgot to convert over the user we already have :)

Oh right, I will convert them over. The fundemental goal of this patch
is to allow a way for vendors to support their own extensions without
needing to populate riscv_isa_ext. This is to create separation between
vendors so they do not impact each other.

xlinuxenvcfg does not fit into this scheme however. This scheme assumes
that a hart cannot have multiple vendors which that extension breaks.
xlinuxenvcfg is really filling a hole in the standard isa that is
applicible to all vendors and does not appear in the device tree so it
is okay for that to live outside this scheme.

> 
> > It is also unecessary to push off the refactoring until we have some
> > "sufficient" amount of vendor extensions to deem changing the
> > infrastructure when I already have the patch available here. This does
> > not introduce any extra overhead to existing functions and will be able
> > to support vendors into the future.
> 
> Yeah, maybe that's true but this was my gut reaction before reading the
> other patch in detail (which I've still yet to do).

- Charlie


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