linux-kernel - Re: [RFC 1/2] virtio: Add AMBA bus driver for virtio device

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Message-ID: <1315906832.3297.41.camel@hornet.cambridge.arm.com>
Date:	Tue, 13 Sep 2011 10:40:32 +0100
From:	Pawel Moll <pawel.moll@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Anthony Liguori <aliguori@...ibm.com>,
	"Michael S.Tsirkin" <mst@...hat.com>,
	Magnus Damm <magnus.damm@...il.com>,
	linux-kernel@...r.kernel.org,
	virtualization <virtualization@...ts.linux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 1/2] virtio: Add AMBA bus driver for virtio device

> > No problem, I've reserved 128 bits in the modified registers layout (see
> > RFC v2), I can even change it into "offset/value" interface, which would
> > give you variable width. One thing I don't get is how to access feature
> > bits > 31? The get_features() seems
> > to be limited to 32:
> > 
> > struct virtio_config_ops {
> > <...>
> > 	u32 (*get_features)(struct virtio_device *vdev);
> > <...>
> > }
> 
> There's a patch for that already :)

Ok, so how about one of the following two options:

1. 4 * 32-bit words of feature flags, 128 bits in total:

 *  0x010   32  HostFeatures0  Features supported by the host, bits 0-31
 *  0x014   32  HostFeatures1  Features supported by the host, bits 32-63
 *  0x018   32  HostFeatures2  Features supported by the host, bits 64-95
 *  0x01c   32  HostFeatures3  Features supported by the host, bits 96-127

 *  0x020   32  GuestFeatures0 Features activated by the guest, bits 0-31
 *  0x024   32  GuestFeatures1 Features activated by the guest, bits 32-63
 *  0x028   32  GuestFeatures2 Features activated by the guest, bits 64-95
 *  0x02c   32  GuestFeatures3 Features activated by the guest, bits 96-127

2. "Select word -> read flags" interface:

 *  0x010   32  HostFeaturesFlags  Features supported by the host
 *  0x014   32  HostFeaturesWord   Set of features to access

 *  0x020   32  GuestFeaturesFlags Features activated by the guest
 *  0x024   32  GuestFeaturesWord  Set of features to set

In this case the algorithm would be:

	writel(0, HostFeaturesWord);
	bits_0_31 = readl(HostFeaturesFlags);
	writel(1, HostFeaturesWord);
	bits_32_63 = readl(HostFeaturesFlags);
	<etc>

The latter seems more "future-proof" to me, if slightly more complex...
Any other suggestions?

> > > > + *  0x008   32  QueuePFN      PFN for the currently selected queue
> > > > + *  0x00c   32  QueueNum      Queue size for the currently selected queue
> > > 
> > > You should, I believe, seriously consider allowing the guest to set the
> > > queue size, rather than the host (perhaps the host could suggest one,
> > > but the guest should be able to override it).
> > 
> > I guess the QueueNum could be simply a read/write register - guest can
> > read suggestion and override it writing it back. The same question
> > though - how can guest change it? (I mean some API?)
> 
> We can sort that out later... the Guest may try some ambitious large
> allocation and fall back if it fails.

So, to my mind, the negotiations could look like this (from the Guest's
point of view):

1. (optional) The Guest is asking what size is "suggested" by the Host:

	size = readl(QueueNum);

2. The Guest requests size it would like to use:

	writel(optimal_size(size), QueueNum);

3. The Host does the best it can to accommodate the Guest and the Guest
checks the effective size:

	size = real(QueueNum);

Does it make some sense?

> > > Anthony or Michael might suggest other changes, since they are most
> > > familiar with virtio_pci limitations...
> > 
> > I've added them on the RFC v2 Cc as well - all feedback more then
> > welcome!
> 
> Excellent...  of course, the virtualization list is down at the moment,
> making this a bit more awkward.

Haha - at least once I can't blame our corporate IT for making my job
harder ;-)

Cheers!

Paweł

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