Re: [PATCH] linux-user: Expose risc-v V and H isa bit in get_elf_hwcap()

[Palmer Dabbelt]

On Wed, 04 May 2022 08:10:03 PDT (-0700), alistair23@gmail.com wrote:
> On Wed, May 4, 2022 at 2:32 PM nihui <shuizhuyuanluo@126.com> wrote:
> > This patch brings the optional risc-v vector and hypervisor bits
> > in hwcap so that application could detect these isa support from
> > /proc/self/auxv correctly in qemu userspace mode.
> >
> > Signed-off-by: Ni Hui <shuizhuyuanluo@126.com>
> > ---
> >  linux-user/elfload.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> > index 61063fd974..3f0ef2b8f6 100644
> > --- a/linux-user/elfload.c
> > +++ b/linux-user/elfload.c
> > @@ -1484,7 +1484,8 @@ static uint32_t get_elf_hwcap(void)
> >  #define MISA_BIT(EXT) (1 << (EXT - 'A'))
> >      RISCVCPU *cpu = RISCV_CPU(thread_cpu);
> >      uint32_t mask = MISA_BIT('I') | MISA_BIT('M') | MISA_BIT('A')
> > -                    | MISA_BIT('F') | MISA_BIT('D') | MISA_BIT('C');
> > +                    | MISA_BIT('F') | MISA_BIT('D') | MISA_BIT('C')
> > +                    | MISA_BIT('V') | MISA_BIT('H');
>
> The kernel doesn't support H or V. I understand V should be supported
> in the future, but what is the use case for H?

IMO even V is a bit in question: sure that bit's likely to be set at 
some point, but there's many flavors of V now and we'll have to give 
userspace a way to differentiate between them.  There's been some 
proposals (see Kito's talk from Plumbers last year, for example) about 
how to deal with this, but nothing really concrete has shown up yet.

If we flip on the V bit in user mode emulation then we run the risk of 
having a wacky ABI here, where QEMU is setting the V bit but then not 
setting whatever extra info is expected to come along with it.  That'd 
mean userspace has to deal with that case -- maybe that's not the worst 
problem, and I guess it's better than just assuming V is always on, 
which is all userspace can do now, but any ABI divergence is going to 
lead to headaches at some point.

IMO the right way forward here is to just sort out what the actual 
interface is, last time I talked to Kito about it we had a rough idea of 
where to go and plans to do it.  Not sure what's up these days, so I've 
added him to the thread.  If it's a long way off then we can always toss 
some intermediate thing together like this, but if it's close then it's 
probably best to just get the interface ironed out and then have it 
match.

> Alistair
>
> >      return cpu->env.misa_ext & mask;
> >  #undef MISA_BIT
> > --
> > 2.25.1