Re: [PATCH v5 2/3] system/memory: Use qemu_ram_move() for directly accessible regions

On Wed, 29 Jul 2026 at 23:35, Michael S. Tsirkin <[email protected]> wrote:
>
> On Tue, Jul 28, 2026 at 01:17:30PM +1000, Gavin Shan wrote:
> > +void qemu_ram_move(void *dst, const void *src, size_t n)
> > +{
> > +    uintptr_t test, len;
> > +
> > +    if (src == dst || n == 0) {
> > +        return;
> > +    }
> > +
> > +    /*
> > +     * Maximal length of aligned access that are determined by @src,
> > +     * @dst and @n
> > +     */
> > +    test = (uintptr_t)src | (uintptr_t)dst | n;
> > +    len = test & -test;
> > +
> > +    /* Overlapping buffers, unaligned or oversized access */
> > +    if (n > 8 || len != n) {
> > +        memmove(dst, src, n);
> > +        return;
> > +    }
>
>
> This part I don't get exactly. It's fine on most arches I think.
> x86, power...
>
> But here is aarch64 for example:
> https://codebrowser.dev/glibc/glibc/sysdeps/aarch64/memcpy.S.html
>
> it says
>    It uses unaligned accesses and branchless sequences to keep the code small,
>    simple and improve performance.
>
> so if the point is to get rid of unaligned accesses to BARs, are you sure
> it achieves this?

The idea is rather just to ignore the unaligned-access-to-not-RAM
case as "only a theoretical issue" and default it to memmove().
If anybody reports an actual problem we can think about it then :-)

-- PMM