Re: [PATCH v4 07/17] x86/hvm: add length to mmio check op

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 25 June 2015 14:48
> To: Paul Durrant
> Cc: Andrew Cooper; xen-devel@lists.xenproject.org; Keir (Xen.org)
> Subject: RE: [PATCH v4 07/17] x86/hvm: add length to mmio check op
> 
> >>> On 25.06.15 at 15:36, <Paul.Durrant@citrix.com> wrote:
> > I think that also allows me to simplfy the patch since I don't have to
> > modify the mmio_check op any more. I simply call it once for the first byte
> > of the access and, if it accepts, verify that it also accepts the last byte
> > of the access.
> 
> That's actually not (generally) okay: There could be a hole in the
> middle. But as long as instructions don't do accesses wider than
> a page, we're fine with that in practice I think. Or wait, no, in the
> MSI-X this could not be okay: A 64-byte read to the 16 bytes
> 32 bytes away from a page boundary (and being the last entry
> on one device's MSI-X table) would extend into another device's
> MSI-X table on the next page. I.e. first and last bytes would be
> okay to be accessed, but bytes 16...31 of the access wouldn't.
> Of course the MSI-X read/write handlers don't currently permit
> such wide accesses, but anyway...
> 

We could also verify that, for a rep op, all reads/writes come back with OKAY. I think that would be ok.

  Paul

> Jan