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

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Message-ID: <20130117110829.GA21859@u109add4315675089e695.ant.amazon.com>
Date:	Thu, 17 Jan 2013 13:22:47 +0100
From:	Matt Wilson <msw@...zon.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xensource.com>,
	<annie.li@...cle.com>
Subject: Re: [PATCH] xen/grant-table: Force to use v1 of grants.

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.

Matt

> Oracle-bug: 16039922
> CC: annie.li@...cle.com
> CC: msw@...zon.com
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  drivers/xen/grant-table.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 95ce9d0..568fd8f 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -1108,10 +1108,8 @@ static void gnttab_request_version(void)
>  	int rc;
>  	struct gnttab_set_version gsv;
>  
> -	if (xen_hvm_domain())
> -		gsv.version = 1;
> -	else
> -		gsv.version = 2;
> +	gsv.version = 1;
> +
>  	rc = HYPERVISOR_grant_table_op(GNTTABOP_set_version, &gsv, 1);
>  	if (rc == 0 && gsv.version == 2) {
>  		grant_table_version = 2;
> -- 
> 1.8.0.2
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