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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@xxxxxxxxxx>  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.

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