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

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Date:	Thu, 17 Jan 2013 21:32:35 -0500
From:	konrad wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Matt Wilson <msw@...zon.com>, 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 1/17/2013 8:05 AM, Jan Beulich wrote:
>>>> 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...

That was the idea, but the work on that stalled. I think that once the 
netfront or netback has the right
code, then we can revert this patch - or just make this be more 
automatic and change itself to V2 if
it can.
> Jan
>
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