linux-kernel - Re: [PATCHv2] kvm: remove in_range and switch to rwsem for iobus

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Message-ID: <20090629094146.GC19167@redhat.com>
Date:	Mon, 29 Jun 2009 12:41:46 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	paulmck@...ux.vnet.ibm.com, markmc@...hat.com
Subject: Re: [PATCHv2] kvm: remove in_range and switch to rwsem for iobus

On Mon, Jun 29, 2009 at 11:37:00AM +0300, Avi Kivity wrote:
> On 06/28/2009 10:34 PM, Michael S. Tsirkin wrote:
>> This changes bus accesses to use high-level kvm_io_bus_read/kvm_io_bus_write
>> functions, which utilize read/write semaphore intead of mutex.  in_range now
>> becomes unused so it is removed from device ops in favor of read/write
>> callbacks performing range checks internally.
>>
>> This allows aliasing (mostly for in-kernel virtio), as well as better error
>> handling by making it possible to pass errors up to userspace. And it's enough
>> to look at the diffstat to see that it's a better API anyway.
>>
>> While we are at it, document locking rules for kvm_io_device_ops.
>>
>> Note: since the use of the new bus_lock is localized to a small number of
>> places, it will be easy to switch to srcu in the future if we so desire.
>>    
>
> Looks good. But please split into a locking change patch and an API  
> change patch (in whatever order makes more sense).

This is harder than it seems. Is this really important?

The locking change itself is about 6 lines, but
1. if I do it after in_range removal I get deadlocks
as after marcelo's change kvm->lock is taken internally by writers.
2. if I do it before in_range removal it's a lot of churn:
one of the reasons for code reorg is so that there are less
places to change locking.


> I think you can reuse slots_lock instead of adding a new lock. IIRC  
> slots_lock is already taken for read everywhere, so you only need to  
> take it for write when registering things.
>
> -- 
> error compiling committee.c: too many arguments to function
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