Re: [Qemu-devel] [PULL 1/7] exec: Limit translation limiting in address_space_translate to xen

[Paolo Bonzini]

Il 07/05/2014 17:12, Peter Maydell ha scritto:
> > > -    if (memory_access_is_direct(mr, is_write)) {
> > > +    if (xen_enabled() && memory_access_is_direct(mr, is_write)) {
> > >          hwaddr page = ((addr & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE) - addr;
> > >          len = MIN(page, len);
> > >      }
> We should put this patch in (both as an efficiency thing and an
> expedient fix) but we really need to either track down which
> callers of this API are relying on the returned plen not being
> truncated, or we need to fix Xen to not truncate either. This
> is just a bandaid IMHO.

Fixing Xen to not truncate is not possible because of the Xen mapcache, 
unless of course QEMU is changed to avoid the mapcache completely on 
64-bit hosts.  I'm not sure if that makes sense from the Xen point of view.

Regarding fixing callers, a known one is virtio-scsi which is a bug and 
on my todo list.  But another is VFIO, which cannot accept truncation if 
the IOMMU page size is greater than TARGET_PAGE_SIZE.

Paolo