Re: [PATCH] RISC-V: Enable Zicbom in usermode

[Palmer Dabbelt]

On Fri, 25 Oct 2024 03:16:44 PDT (-0700), Conor Dooley wrote:
> On Fri, Oct 25, 2024 at 05:15:27PM +0800, Yunhui Cui wrote:
> > Like Zicboz, by enabling the corresponding bits of senvcfg,
> > the instructions cbo.clean, cbo.flush, and cbo.inval can be
> > executed normally in user mode.
> >
> > Signed-off-by: Yunhui Cui <cuiyunhui@xxxxxxxxxxxxx>
> > ---
> >  arch/riscv/kernel/cpufeature.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index 1992ea64786e..bc850518ab41 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -924,7 +924,7 @@ unsigned long riscv_get_elf_hwcap(void)
> >  void __init riscv_user_isa_enable(void)
> >  {
> >  	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ))
> > -		current->thread.envcfg |= ENVCFG_CBZE;
> > +		current->thread.envcfg |= ENVCFG_CBIE | ENVCFG_CBCFE | ENVCFG_CBZE;
>
> I believe we previously decided that userspace should not be allowed to

Ya, we didn't want to expose this because it opens up a can of worms.  
Is there a use case for this?  It's not like this is entirely impossible 
to do, it just requires a bit of thought (and should probably be gated 
behind some per-process disabling).

> use zicbom, but that not withstanding - this is wrong. It should be
> checking for Zicbom, not Zicboz.