RE: [Xen-devel] [PATCH] xen: grant-table: Check truncation when giving access to a frame

[Paul Durrant]

> -----Original Message-----
> From: Julien Grall [mailto:julien.grall@xxxxxxx]
> Sent: 13 June 2016 13:42
> To: Paul Durrant; boris.ostrovsky@xxxxxxxxxx; David Vrabel;
> jgross@xxxxxxxx; sstabellini@xxxxxxxxxx; konrad.wilk@xxxxxxxxxx
> Cc: Andrew Cooper; xen-devel@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> JBeulich@xxxxxxxx; steve.capper@xxxxxxx
> Subject: Re: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
> giving access to a frame
> 
> 
> 
> On 13/06/16 13:41, Julien Grall wrote:
> > Hello Paul,
> >
> > On 13/06/16 13:12, Paul Durrant wrote:
> >>> -----Original Message-----
> >>> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf
> Of
> >>> Julien Grall
> >>> Sent: 13 June 2016 11:51
> >>> To: boris.ostrovsky@xxxxxxxxxx; David Vrabel; jgross@xxxxxxxx;
> >>> sstabellini@xxxxxxxxxx; konrad.wilk@xxxxxxxxxx
> >>> Cc: steve.capper@xxxxxxx; Andrew Cooper; linux-
> kernel@xxxxxxxxxxxxxxx;
> >>> xen-devel@xxxxxxxxxxxxx; Julien Grall; JBeulich@xxxxxxxx
> >>> Subject: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
> >>> giving
> >>> access to a frame
> >>>
> >>> The version 1 of the grant-table protocol only supports frame encoded
> on
> >>> 32-bit.
> >>>
> >>> When the platform is supporting 48-bit physical address, the frame will
> >>> be encoded on 36-bit which will lead a truncation and give access to
> >>> the wrong frame.
> >>>
> >>> On ARM Xen will always allow the guest to use all the physical address,
> >>> although today the RAM is always located under 40-bits (see
> >>> xen/include/public/arch-arm.h).
> >>>
> >>> Add a truncation check in gnttab_update_entry_v1 to prevent the guest
> to
> >>> give access to the wrong frame.
> >>>
> >>> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> >>>
> >>> ---
> >>>      This is limiting us to a 44-bit address space whilst ARM can
> >>> support
> >>>      up to 48-bit today. This number of bit will increase to 52-bit in
> >>>      upcoming processors [1].
> >>>
> >>>      It might be good to start thinking to extend the version 1 of the
> >>>      protocol to use 64-bit frame number.
> >>
> >> ...or simply use version 2 of the protocol.
> >
> > On another mail [1], you said that "[v2] didn't scale it became
> > bottle-necked on dom0's grant table size,...".
> >
> > So it looks like to me that version 2 is the wrong way to go.
> > The performance should stay the same whether the platform support
> > 40-bit, 44-bit, 48-bit, 52-bit address space.
> 

No, I meant the guest receive-side copy didn't scale, not grant table v2 itself. Ok the table is bigger with v2, but to do guest receive-side copy required a huge table in dom0 if it was going to scale to 100s of VMs and the perf. benefits were never that great (if they were there at all).

  Paul

> I forgot the link.
> 
> [1]
> http://lists.xenproject.org/archives/html/xen-devel/2016-06/msg01606.html
> 
> --
> Julien Grall