Re: RISCV Vector unit disabled by default for new task (was Re: [PATCH v12 17/17] riscv: prctl to enable vector commands)

[Icenowy Zheng <uwu@icenowy.me>]

在 2022-12-08星期四的 22:27 -0800，Palmer Dabbelt写道：
> On Thu, 08 Dec 2022 21:16:06 PST (-0800), Vineet Gupta wrote:
> > Hi Darius, Andrew, Palmer
> > 
> > On 9/21/22 14:43, Chris Stillson wrote:
> > > diff --git a/arch/riscv/kernel/process.c
> > > b/arch/riscv/kernel/process.c
> > > 
> > > @@ -134,7 +135,6 @@ void start_thread(struct pt_regs *regs,
> > > unsigned long pc,
> > >                         if (WARN_ON(!vstate->datap))
> > >                                 return;
> > >                 }
> > > -               regs->status |= SR_VS_INITIAL;
> > > 
> > 
> > Perhaps not obvious from the patch, but this is a major user
> > experience
> > change: As in V unit would be turned off for a new task and we will
> > rely
> > on a userspace prctl (also introduced in this patch) to enable V.
> 
> IMO that's the only viable option: enabling V adds more user-visible 
> state, which is a uABI break.  I haven't really had time to poke
> through 
> all the versions here, but I'd have the call look something like
> 
>     prctl(RISCV_ENABLE_V, min_vlenb, max_vlenb, flags);

Should we make this extra switch more future-proof by not only limiting
it to V, but also other extensions that will introduce extra state,
e.g. P ?

> 
> where
> 
> * min_vlenb is the smallest VLENB that userspace can support. 
> There's 
>   alreday an LLVM argument for this, I haven't dug into the generated
>   code but I assume it'll blow up on smaller VLENB systems somehow.
> * max_vlenb is the largest VLENB that userspace can support.
> * flags is just a placeholder for now, with 0 meaning "V as defined
> by 
>   1.0 for all threads in this proces".  That should give us an out if
>   something more complicated happens in the future.
> 
> That way VLA code can call `prctl(RISCV_ENABLE_V, 128, 8192, 0)` as
> it 
> supports any V 1.0 implementation, while code with other constraints
> can 
> avoid having V turned on in an unsupported configuration.
> 
> I think we can start out with no flags, but there's a few I could see
> being useful already:
> 
> * Cross process/thread enabling.  I think a reasonable default is 
>   "enable V for all current and future threads in this process", but
> one 
>   could imagine flags for "just this thread" vs "all current
> threads", a 
>   default for new threads, and a default for child processes.  I
> don't 
>   think it matters so much what we pick as a default, just that it's 
>   written down.
> * Setting the VLENB bounds vs updating them.  I'm thinking for shared
>   libraries, where they'd only want to enable V in the shared library
> if 
>   it's already in a supported configuration.  I'm not sure what the 
>   right rules are here, but again it's best to write that down.
> * Some way to disable V.  Maybe we just say `prctl(RISCV_ENABLE_V, 0,
> 0, 
>   ...)` disables V, or maybe it's a flag?  Again, it should just be 
>   written down.
> * What exactly we're enabling -- is it the V extension, or just the V
>   registers?
> 
> There's a bunch of subtly here, though, so I think we'd at least want
> glibc and gdb support posted before committing to any uABI.  It's 
> probably also worth looking at what the Arm folks did for SVE: I gave
> it 
> a quick glance and it seems like there's a lot of similarities with
> what 
> I'm suggesting here, but again a lot of this is pretty subtle stuff
> so 
> it's hard to tell just at a glance.
> 
> > I know some of you had different opinion on this in the past [1],
> > so
> > this is to make sure everyone's on same page.
> > And if we agree this is the way to go, how exactly will this be
> > done in
> > userspace.
> > 
> > glibc dynamic loader will invoke the prctl() ? How will it decide
> > whether to do this (or not) - will it be unconditional or will it
> > use
> > the hwcap - does latter plumbing exist already ? If so is it
> > AT_HWCAP /
> > HWCAP2.
> 
> That part I haven't sorted out yet, and I don't think it's sufficient
> to 
> just say "userspace should enable what it can support" because of how
> pervasive V instructions are going to be.
> 
> I don't think we need HWCAP, as userspace will need to call the
> prctl() 
> anyway to turn on V and thus can just use the success/failure of that
> to 
> sort things out.  
> 
> Maybe it's sufficient to rely on some sort of sticky prctl() (or
> sysctl 
> type thing, the differences there would be pretty subtle) and just
> not 
> worry about it, but having some way of encoding this in the ELF seems
> nice.  That said, we've had a bunch of trouble sorting out the ISA 
> encoding in ELFs so maybe it's just not worth bothering?
> 
> > Also for static linked executables, where will the prctl be called
> > from ?
> 
> I guess that's pretty far in the weeds, but we could at least hook
> CRT 
> to insert the relevant code.  We'd really need to sort out how we're 
> going to encode the V support in binaries, though.
> 
> > [1]
> > https://sourceware.org/pipermail/libc-alpha/2021-November/132883.html
> 
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