Re: [PATCH v3 1/2] system/memory: Use qemu_ram_{copy, move}() in ram device region accessors

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 22, 2026 at 10:53:27AM +1000, Gavin Shan wrote:
> On 7/22/26 2:27 AM, Peter Xu wrote:
> > On Tue, Jul 21, 2026 at 03:37:53PM +1000, Gavin Shan wrote:
> > > If Peter is fine with two variants for x86 and non-x86 architectures.
> > > I can post (v4) for further review. That will be something like below
> > > and let me know if there are any other improvements are needed.
> > 
> > I have a generic question on the "unaligned access for x86": I think the
> > question is about the one Michael raised here on unaligned access may break
> > x86 here:
> > 
> >    https://lore.kernel.org/qemu-devel/20260617022330-mutt-send-email-mst@kernel.org/
> > 
> >    3. (theoretical concern) also on x86, unaligned accesses are
> >    possible on guest and host, so converting an unaligned access to a
> >    series of aligned ones can in theory break devices.
> > 
> > Is that a real problem we need to consider, or can we start with unified
> > approach and leave it for later?
> > 
> 
> I'm leaving this question to Michael.

Knowing what I know about hardware designers, it's something someone
somewhere does)
It can be made a separate patch, just to show - it should be all of
~10LOC.


> > PS: I apologize if I missed important piece of info along the way; I didn't
> > follow closely on the discussion on this topic in the past few weeks.
> > 
> > One thing to mention is, what we change should only need to affect
> > ram_device, AFAIU.. so most memcpy()/memmove() shouldn't be changed for any
> > arch when it's pure RAM.
> > 
> 
> It depends. This patch intends to fix issue [1] in the lower layer by using
> the newly added accessors (qemu_ram_{copy, move}) on all directly accessible
> regions including the regular (pure) RAM region. Otherwise, the newly added
> accessors should be limited to ram_device regions only as you said.
> 
> [1] https://lore.kernel.org/qemu-devel/20260527091711.3901-1-liugang24219@sangfor.com.cn/
> 
> Thanks,
> Gavin

using memcpy()/memmove() to emulate guest's atomics is generally
kinda broken.
but yes there are architectures where doing it to device ram is
more broken than doing it to regular ram.


-- 
MST