Re: [PATCH 0/4] riscv: uaccess: optimizations

[Will Deacon <will@kernel.org>]

On Mon, Jul 08, 2024 at 04:30:52PM +0100, Mark Rutland wrote:
> On Mon, Jul 08, 2024 at 02:52:43PM +0100, Will Deacon wrote:
> > On Fri, Jul 05, 2024 at 10:58:29AM -0700, Linus Torvalds wrote:
> > > On Fri, 5 Jul 2024 at 04:25, Will Deacon <will@kernel.org> wrote:
> > > So on x86-64, the simple solution is to just say "we know if the top
> > > bit is clear, it cannot ever touch kernel code, and if the top bit is
> > > set we have to make the address fault". So just duplicating the top
> > > bit (with an arithmetic shift) and or'ing it with the low bits, we get
> > > exactly what we want.
> > > 
> > > But my knowledge of arm64 is weak enough that while I am reading
> > > assembly language and I know that instead of the top bit, it's bit55,
> > > I don't know what the actual rules for the translation table registers
> > > are.
> > > 
> > > If the all-bits-set address is guaranteed to always trap, then arm64
> > > could just use the same thing x86 does (just duplicating bit 55
> > > instead of the sign bit)?
> > 
> > Perhaps we could just force accesses with bit 55 set to the address
> > '1UL << 55'? That should sit slap bang in the middle of the guard
> > region between the TTBRs
> 
> Yep, that'll work until we handle FEAT_D128 where (1UL << 55) will be
> the start of the TTBR1 range in some configurations.
> 
> > and I think it would resolve any issues we may have with wrapping. It
> > still means effectively reverting 2305b809be93 ("arm64: uaccess:
> > simplify uaccess_mask_ptr()"), though.
> 
> If we do bring that back, it'd be nice if we could do that without the
> CSEL+CSDB, as the CSDB is liable to be expensive on some parts (e.g.
> it's an alias of DSB on older designs).

DSB?! Are you sure? I thought it was basically a NOP for everybody apart
from a small subset of implementations.

Will