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

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Date:	Mon, 13 Jun 2016 13:14:22 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Julien Grall <julien.grall@....com>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"jgross@...e.com" <jgross@...e.com>,
	"sstabellini@...nel.org" <sstabellini@...nel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
CC:	Andrew Cooper <Andrew.Cooper3@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"steve.capper@....com" <steve.capper@....com>
Subject: RE: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
 giving access to a frame

> -----Original Message-----
[snip]
> >
> > 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).
> 
> Sorry I misunderstood your previous mail. So the only downside is the
> size of the table.
> 
> Looking at the structure in the header (public/grant_table.h), this is
> effectively much bigger. A commit in Linux [1] suggests that grant v2
> only supports 256 grants per page rather than 512 for v1.
> 

That's correct.

> How would that impact a guest?
> 

Well, for the same table size you'd get half as many refs.

If you don't have many vifs and vbds in your guest then you'll probably be fine. If you do then you might see pressure for free entries and some drivers may actually crash if they can't get what they want. I don't believe the size of the guest's grant table can be tuned by the toolstack... probably about time to add that capability.

  Paul

> Regards,
> 
> [1] commit 11c7ff17c9b6dbf3a4e4f36be30ad531a6cf0ec9
> Author: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Date:   Mon Jan 6 10:44:39 2014 -0500
>      xen/grant-table: Force to use v1 of grants.
> 
>      We have the framework to use v2, but there are no backends that
>      actually use it. The end result is that on PV we use v2 grants
>      and on PVHVM v1. The v1 has a capacity of 512 grants per page while
>      the v2 has 256 grants per page. This means we lose about 50%
>      capacity - and if we want more than 16 VIFs (each VIF takes
>      512 grants), then we are hitting the max per guest of 32.
> 
> So from my understanding the table is much more bigger.
> 
> --
> Julien Grall