Am 06.08.2010 um 11:34 schrieb "Ira W. Snyder" [off-list ref]:

On Fri, Aug 06, 2010 at 02:20:42AM +0300, Michael S. Tsirkin wrote:

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On Thu, Aug 05, 2010 at 04:01:03PM -0700, Ira W. Snyder wrote:

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On Fri, Aug 06, 2010 at 12:30:50AM +0300, Michael S. Tsirkin wrote:

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Hi Ira,

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Making my life harder since the last time I tried this, mainline commit
7c5e9ed0c (virtio_ring: remove a level of indirection) has removed the
possibility of using an alternative virtqueue implementation. The commit
message suggests that you might be willing to add this capability back.
Would this be an option?

Sorry about that.

With respect to this commit, we only had one implementation upstream
and extra levels of indirection made extending the API
much harder for no apparent benefit.

When there's more than one ring implementation with very small amount of
common code, I think that it might make sense to readd the indirection
back, to separate the code cleanly.

OTOH if the two implementations share a lot of code, I think that it
might be better to just add a couple of if statements here and there.
This way compiler even might have a chance to compile the code out if
the feature is disabled in kernel config.

The virtqueue implementation I envision will be almost identical to the
current virtio_ring virtqueue implementation, with the following
exceptions:

* the "shared memory" will actually be remote, on the PCI BAR of a device
* iowrite32(), ioread32() and friends will be needed to access the memory
* there will only be a fixed number of virtqueues available, due to PCI
 BAR size
* cross-endian virtqueues must work
* kick needs to be cross-machine (using PCI IRQ's)

I don't think it is feasible to add this to the existing implementation.
I think the requirement of being cross-endian will be the hardest to
overcome. Rusty did not envision the cross-endian use case when he
designed this, and it shows, in virtio_ring, virtio_net and vhost. I
have no idea what to do about this. Do you have any ideas?

My guess is sticking an if around each access in virtio would hurt,
if this is what you are asking about.

Yes, I think so too. I think using le32 byte order everywhere in virtio
would be a good thing. In addition, it means that on all x86, things
continue to work as-is. It would also have no overhead in the most
common case: x86-on-x86.

This problem is not limited to my new use of virtio. Virtio is
completely useless in a relatively common virtualization scenario:
x86 host with qemu-ppc guest. Or any other big endian guest system.

This one actually works because we know that we're building for a BE guest. But I agree that it's a mess and clearly a very incorrect design decision.

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Just a crazy idea: vhost already uses wrappers like get_user etc,
maybe when building kernel for your board you could
redefine these to also byteswap?

I think idea is clever, but also psychotic :) I'm sure it would work,
but that only solves the problem of virtio ring descriptors. The
virtio-net header contains several __u16 fields which would also need
to be fixed-endianness.

I'd vote for defining virtio v2 that makes everything LE. Maybe we could even have an LE capability with a grace period of phasing out non-LE capable hosts and guests.


Alex