Re: [PATCH v2] virtio: Add aligned ld/st accessors for vring

[Richard Henderson <richard.henderson@linaro.org>]

On 8/21/26 03:25, Peter Maydell wrote:
> I'm tempted to suggest some kind of "if pointer is aligned take
> aligned path, otherwise take slow path" either here or actually
> in lduw_le_p(), but maybe that's a bad idea. Richard ?
> 
> (I have a suspicion that other places than this one will assume
> that an aligned ldl_he_p() is not going to tear.)
I agree -- I expect most everything assumes ldl_he_p won't tear for aligned accesses.

This kinda begs the question of what atomicity the caller expects.  It's not implausible 
that an x86 path expects even unaligned accesses not crossing a cacheline to be atomic, 
since that's been a thing since 1995.  I expect both IBM architectures similarly expect 
atomicity by alignment, since that's been a thing for s390 since yonks and Power has the 
same language.

We have a bunch of code in accel/tcg/ldst_atomicity.c.inc that can handle this, we'd just 
need to provide it with the correct inputs.  And I assume we'd still like to inline the 
single access on appropriate hosts.


r~