linux-kernel - Re: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors

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Date:	Tue, 5 Mar 2013 18:00:51 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Jan Beulich <JBeulich@...e.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors

On 05/03/13 15:16, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 05, 2013 at 08:11:19AM +0000, Jan Beulich wrote:
>>>>> On 04.03.13 at 21:44, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
>>> <nods> 'op' sounds good. With a comment saying it can do all of the 
>>> BLKIF_OPS_..
>>> except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
>>> it that way for fun.
>>
>> In fact I'd like to exclude chaining as well as BLKIF_OP_DISCARD here.
>> The former should - if useful for anything - be controlled by a
>> separate feature flag, and the latter is plain pointless to indirect.
>> And I reckon the same would apply to BLKIF_OP_FLUSH_DISKCACHE
>> and BLKIF_OP_RESERVED_1 - i.e. it might be better to state that
>> indirection is only permitted for normal I/O (read/write) ops.
> 
> <nods> That makes sense. And also of course the new BLKIF_OP should
> be documented in the Xen tree as well.

The only ops that can be done indirectly are _READ, _WRITE and
_BARRIER/_FLUSH. From the implementation in blkfront it seems like
_FLUSH/_BARRIER requests can indeed contain segments, but I haven't been
able to spot any _FLUSH op with segments on it. Can you confirm FLUSH
requests never contain bios?
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