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
