linux-kernel - Re: [Xen-devel] [PATCH] xen/grant-table: Force to use v1 of grants.

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Date:	Thu, 17 Jan 2013 13:05:00 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Matt Wilson" <msw@...zon.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	<xen-devel@...ts.xensource.com>, <annie.li@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/grant-table: Force to use v1 of
 grants.

>>> On 17.01.13 at 13:22, Matt Wilson <msw@...zon.com> wrote:
> On Wed, Jan 16, 2013 at 04:22:49PM -0500, Konrad Rzeszutek Wilk wrote:
>> 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.
>> 
>> Note: The hypervisor provides an over-ride for the default
>> of 32 frames (pages) per guest: gnttab_max_nr_frames=X
> 
> I think it would be better if the dom0/service domain and hypervisor
> prevent the guest from setting the version to 2 if the backend isn't
> going to make use of them. In the short term, we could add a Xen
> option to limit the supported version to 1.

But that's something each guest has to decide for itself.

Anyway - if no-one's using v2, what did it get introduced for
(not only in the hypervisor, but also in the kernel)? I thought
the networking drivers were intended to make use of it...

Jan

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