linux-kernel - Re: [PATCH RFC] Persistent grant maps for xen blk drivers

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Date:	Tue, 23 Oct 2012 20:09:27 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Konrad Rzeszutek Wilk <konrad@...nel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] Persistent grant maps for xen blk drivers

On 23/10/12 19:20, Konrad Rzeszutek Wilk wrote:
>>>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>>>> index c6decb9..2b982b2 100644
>>>> --- a/drivers/block/xen-blkback/blkback.c
>>>> +++ b/drivers/block/xen-blkback/blkback.c
>>>> @@ -78,6 +78,7 @@ struct pending_req {
>>>>       unsigned short          operation;
>>>>       int                     status;
>>>>       struct list_head        free_list;
>>>> +     unsigned int            unmap_seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];

Should I change this to a bool? Since we are only setting it to 0 or 1.

>>> Perhaps there should be a #define for that array..
>>
>> Do you mean something like:
>>
>> #define unmap(req, i) req->unmap_seg[i]
> 
> I was thinking that you just check for req->unamp_seg[i] to
> have an non-zero value. But since that array is just used as an check
> to see whether the functionality is enabled (or not), you might want
> to declerare the right values so:
> #define UNMAP_SG_ON 1
> #define UNMAP_SG_OFF 0
> 
> or so.

Agreed, will add the defines.

>>>> +             if (persistent_gnts[i]) {
>>>> +                     if (!persistent_gnts[i]->handle) {
>>>> +                             /*
>>>> +                              * If this is a new persistent grant
>>>> +                              * save the handler
>>>> +                              */
>>>> +                             persistent_gnts[i]->handle = map[j].handle;
>>>> +                             persistent_gnts[i]->dev_bus_addr =
>>>> +                                     map[j++].dev_bus_addr;
>>>> +                     }
>>>> +                     pending_handle(pending_req, i) =
>>>> +                             persistent_gnts[i]->handle;
>>>> +                     pending_req->unmap_seg[i] = 0;
>>>
>>> Could we have a #define for that?
>>
>> Sure.

I've used the previous macro, so it looks like:

unmap(req, i) = UNMAP_SG_OFF;

I'm not sure if this is what you meant, or if you where interested in
defining a set of macros like:

#define check_unmap(req, i) req->unmap_seg[i]
#define unset_unmap(req, i) req->unmap_seg[i] = UNMAP_SG_OFF
#define set_unmap(req, i) req->unmap_seg[i] = UNMAP_SG_ON

I would go for the first option (the unmap macro that can be used here
and in xen_blkbk_unmap).

>>> HA! By default, eh?
>>
>> Yes, you caught me, there's a paragraph in the commit message that
>> explains that we are using persistent grants in the frontend
>> unconditionally, since the protocol is compatible (you can have a
>> persistent blkfront and a non-persistent blkback). It simplifies the
>> logic in blkfront. Are you OK with it?
> 
> It is OK, but you should be checking whether the backend supports it.
> I don't see it checking the info->feature_persistent_grant to print
> that.

I don't understand why blkfront needs to check if the backend supports
persisten grants, blkfront is going to use persistent grants anyway.

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