Re: [Xen-devel] [PATCH] xen: blkif.h: document linux xen-block multi-page ring implementation

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> Sent: 15 May 2015 16:55
> To: Jan Beulich
> Cc: Bob Liu; msw@xxxxxxxxxx; David Vrabel; Paul Durrant; Roger Pau
> Monne; xen-devel@xxxxxxxxxxxxx; justing@xxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [Xen-devel] [PATCH] xen: blkif.h: document linux xen-block
> multi-page ring implementation
> 
> On Fri, May 15, 2015 at 04:38:58PM +0100, Jan Beulich wrote:
> > >>> On 15.05.15 at 17:22, <konrad.wilk@xxxxxxxxxx> wrote:
> > > Doing an git annotate shows:
> > >
> > > commit 829f2a9c6dfae13faee4e24b953441113338e0f2
> > > Author: Justin T. Gibbs <justing@xxxxxxxxxxxxxxxx>
> > > Date:   Thu Feb 23 10:02:30 2012 +0000
> > >
> > >     blkif.h: Document the Red Hat and Citrix blkif multi-page ring
> extensions
> > >
> > >     No functional changes.
> > >
> > >
> > > Which says:
> > >
> > >  * (1) Multi-page ring buffer scheme first developed in the Citrix
> XenServer
> > > + *     PV drivers.
> > > + * (2) Multi-page ring buffer scheme first used in some RedHat
> distributions
> > > + *     including a distribution deployed on certain nodes of the Amazon
> > > + *     EC2 cluster.
> > > + * (3) Support for multi-page ring buffers was implemented
> independently,
> > > + *     in slightly different forms, by both Citrix and RedHat/Amazon.
> > >
> > >
> > > And I double checked the linux-2.6.18.hg tree and I see no mention of
> > > multi-page. I wonder where such code exists?
> >
> > We have support for multi page rings in our kernels, largely because
> > (iirc) Amazon had asked it.
> 
> Aha!
> 
> Bob has found that he can use 'ring-page-order' and 'max-ring-page-order'
> which are the new standard and not have to worry about breaking older
> guests.

The Xen Project Windows drivers use those keys too: see 
http://xenbits.xen.org/gitweb/?p=pvdrivers/win/xenvbd.git;a=blob;f=src/xenvbd/blockring.c;h=35ffb3b39d1478f08fbe016f992de152a494c0e1;hb=HEAD

  Paul

> >
> > Jan
> >

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