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

[yunhui cui]

Hi Jessica,

On Sat, Oct 26, 2024 at 12:32 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>
> On 25 Oct 2024, at 11:16, Conor Dooley <conor at kernel.org> 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 at bytedance.com>
> >> ---
> >> 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
> > use zicbom, but that not withstanding - this is wrong. It should be
> > checking for Zicbom, not Zicboz.
>
> Allowing clean/flush is safe but has the same problems as fence.i with
> regards to migrating between harts. Allowing invalidate, unless mapped
> to flush, is not safe in general unless the kernel does a lot of
> flushing to avoid userspace accessing data it shouldn’t be able to see.
>
> Also, ENVCFG_CBIE is a mask for a multi-bit field, which happens to
> have the same value as ENVCFG_CBIE_INV (i.e. really is making cbo.inval
> be an invalidate). I note that the KVM code, which this likely copied
> from(?), makes the same mistake, but there that is the intended
> behaviour, if misleading about what the field really is.
>
> So, with suitable caveats, allowing clean/flush could be a reasonable
> thing to do (maybe useful for userspace drivers so long as they pin
> themselves to a specific hart?), but invalidate should only ever be
> allowed if mapped to flush.
>
> Jess
>
Yes. The original intention is to enable clean/flush/invalid. So
ENVCFG_CBIE | ENVCFG_CBCFE is added. When one core initiates an
invalidation, other cores will also invalidate the corresponding cache
line. So do we not need to worry about this problem? Moreover,
invalidation is not found in the logic of disabling preemption in the
kernel. Or perhaps binding cores belongs to the user-space's own
logic. Can this patch be fixed as RISCV_ISA_EXT_ZICBOM and then a v2
be sent?

Thanks,
Yunhui