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Date: Wed, 07 May 2014 17:22:35 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PULL 1/7] exec: Limit translation limiting in
 address_space_translate to xen
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To: Peter Maydell <peter.maydell@linaro.org>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Olaf Hering <olaf@aepfle.de>, "xen-devel@lists.xensource.com Devel" <xen-devel@lists.xensource.com>, Alexey Kardashevskiy <aik@ozlabs.ru>, QEMU Developers <qemu-devel@nongnu.org>, Anthony Liguori <anthony@codemonkey.ws>, Anthony PERARD <Anthony.Perard@citrix.com>

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