linux-kernel - Re: [PATCH] virtio config

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Message-ID: <4732774C.6020903@codemonkey.ws>
Date:	Wed, 07 Nov 2007 20:41:16 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Anthony Liguori <aliguori@...ibm.com>, lguest <lguest@...abs.org>,
	linux-kernel@...r.kernel.org, Dor Laor <dor.laor@...ranet.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio config_ops refactoring

Rusty Russell wrote:
> On Thursday 08 November 2007 04:30:50 Anthony Liguori wrote:
>> I would prefer that the virtio API not expose a little endian standard.
>> I'm currently converting config->get() ops to ioreadXX depending on the
>> size which already does the endianness conversion for me so this just
>> messes things up.  I think it's better to let the backend deal with
>> endianness since it's trivial to handle for both the PCI backend and the
>> lguest backend (lguest doesn't need to do any endianness conversion).
> 
> -ETOOMUCHMAGIC.  We should either expose all the XX interfaces (but this isn't 
> a high-speed interface, so let's not) or not "sometimes" convert endianness.  
> Getting surprises because a field happens to be packed into 4 bytes is 
> counter-intuitive.

Then I think it's necessary to expose the XX interfaces.  Otherwise, the 
backend has to deal with doing all register operations at a per-byte 
granularity which adds a whole lot of complexity on a per-device basis 
(as opposed to a little complexity once in the transport layer).

You really want to be able to rely on multi-byte atomic operations too 
when setting values.  Otherwise, you need another register to just to 
signal when it's okay for the device to examine any given register.

Regards,

Anthony Liguori

> Since your most trivial implementation is to do a byte at a time, I don't 
> think you have a good argument on that basis either.
> 
> Cheers,
> Rusty.

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