Re: [PATCH 1/2] system/memory: Use __builtin_mem{cpy, move} in accessors of ram device region

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

On Sun, Jun 14, 2026 at 06:41:57PM -0700, Richard Henderson wrote:
> On 6/14/26 10:22, Michael S. Tsirkin wrote:
> > On Sun, Jun 14, 2026 at 09:45:51AM -0700, Richard Henderson wrote:
> > > On 6/14/26 08:13, Michael S. Tsirkin wrote:
> > > > Yes, I think it does work because we use -fno-strict-aliasing.
> > > > For bigger sizes we'll need packed because the addresses
> > > > could be unaligned.
> > > ...
> > > > For most host/guest pairs things simply work even for unaligned.
> > > > 
> > > > And yes, guest drivers do do this.
> > > > 
> > > > On classical pci, there are no transactions as such and
> > > > an unaligned access will be split anyway.
> > > 
> > > I'm saying, if you're talking about pass-through to real devices, that won't
> > > work. For instance, AArch64 will trap unaligned accesses to Device memory.
> > 
> > Presumably, AArch64 drivers don't do unaligned at all then?
> 
> Yes.
> 
> > > You need to actually handle unaligned.  Perhaps something like
> > > 
> > >      /* Find unit to fit size and alignment of dst */
> > >      uintptr_t test = (uintptr_t)dst | size;
> > >      uintptr_t lsb = test & -test;
> > > 
> > >      switch (lsb) {
> > >      case 1:   // loop over uint8_t
> > >      case 2:   // loop over uint16_t
> > >      case 4:   // loop over uint32_t
> > >      default:  // loop over uint64_t
> > >      }
> > > 
> > > with the expectation that normally we'll have aligned addresses and size
> > > such that the loop will iterate once.
> > > 
> > > 
> > > r~
> > 
> > And ifdef for arches without unaligned support?
> 
> No ifdef.  All accesses produced by the above are aligned.
> 
> 
> r~

So something like:

#if defined(__i386__) || defined(__x86_64__)
#define HOST_UNALIGNED_MMIO_OK  1
#else
#define HOST_UNALIGNED_MMIO_OK  0
#endif


and then we can check that.

-- 
MST