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Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO"

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO"
Date: Thu, 28 May 2015 13:41:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 28/05/2015 13:24, Kevin Wolf wrote:
> Am 28.05.2015 um 13:00 hat Paolo Bonzini geschrieben:
>> On 28/05/2015 12:55, Fam Zheng wrote:
>>>> Indeed. blk_pause/resume would handle everything in one central place
>>>> in the block layer instead of spreading the logic across all the block
>>>> layer users.
>>> Sorry, I'm confused. Do you mean there is a way to implement blk_pause
>>> completely in block layer, without the necessity of various notifier 
>>> handlers
>>> in device models?
>> How would you do that?  Do you have to keep a queue of pending requests
>> in the BlockBackend?  Since bdrv_drain_all may never return (e.g. stuck
>> NFS connection with nfs=hard), the guest can force unbounded allocation
>> in the host, which is bad.
> We already queue requests for things like I/O throttling or
> serialisation. Why would this be any worse?

The fact that it's potentially unbounded makes me nervous.  But you're
right that we sort of expect the guest to not have too high a queue
depth.  And serialization is also potentially unbounded.

So it may indeed be the best way.  Just to double check, is it correct
that the API would still be BDS-based, i.e.
bdrv_pause_backends/bdrv_resume_backends, and the BlockBackends would
attach themselves to pause/resume notifier lists?


>> In addition, the BDS doesn't have a list of BlockBackends attached to
>> it.  So you need the BlockBackends to register themselves for
>> pause/resume in some way---for example with a notifier list.
>> Then it's irrelevant whether it's the device model or the BB that
>> attaches itself to the notifier list.  You can start with doing it in
>> the device models (those that use ioeventfd), and later it can be moved
>> to the BB.  The low-level implementation remains the same.
> The reason for doing it in the block layer is that it's in one place and
> we can be sure that it's applied. We can still in addition modify
> specific users to avoid even trying to send requests, but I think it's
> good to have the single place that always ensures correct functionality
> of the drain instead of making it dependent on the user.
> Kevin

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