linux-kernel - Re: [PATCH] virtio config

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Message-Id: <200711092254.08099.rusty@rustcorp.com.au>
Date:	Fri, 9 Nov 2007 22:54:07 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Anthony Liguori <anthony@...emonkey.ws>
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

On Friday 09 November 2007 09:33:04 Anthony Liguori wrote:
> I really want to make sure that if a guest tries
> to read a 4-byte PCI config field, that it does so using an "outl"
> instruction so that in my QEMU backend

So you want to enforce PCI requirements onto virtio config accesses.  This 
doesn't seem very nice: the fact that PCI accesses use different namespaces 
for different sizes makes sense from a primitive hardware point of view, but 
sucks for software.  Fortunately, if you insist on byte-at-a-time they're the 
same.

> switch (addr) {
> case VIRTIO_BLK_CONFIG_MAX_SEG:
>    return vdev->max_seg & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SEG + 1:
>    return (vdev->max_seg >> 8) & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SEG + 2:
>    return (vdev->max_seg >> 16) & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SEG + 3:
>    return (vdev->max_seg >> 24) & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SIZE:
>    return vdev->max_size & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SIZE + 1:
>    return (vdev->max_size >> 8) & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SIZE + 2:
>    return (vdev->max_size >> 16) & 0xFF;
> case VIRTIO_BLK_CONFIG_MAX_SIZE + 3:
>    return (vdev->max_size >> 24) & 0xFF;
> ...

	struct virtio_blk_config
	{
		uint32_t max_seg, max_size;
	};

	...
	struct virtio_blk_config conf = { vdev->max_seg, vdev->max_size };

	return ((unsigned char *)&conf)[addr];

(Which strongly implies our headers should expose that nominal struct, rather 
than numerical constants).

Rusty.
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