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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] Re: [PATCH] qemu/virtio: move features to an inline function |
Date: | Tue, 11 Aug 2009 11:08:32 -0500 |
User-agent: | Thunderbird 2.0.0.21 (X11/20090320) |
Michael S. Tsirkin wrote:
Let's say we supported virtio-vbus along with virtio-pci. What does virtio_blk_get_features() do to mask out sg_indirect? For all virtio-blk knows, it could be on top of virtio-vbus.So? VIRTIO_RING_F_INDIRECT_DESC applies to all transports. Just clear this bit.
You can have many layers with virtio. device + transport + ringvirtio-vbus would have a different transport and a different ring implementation. So no, VIRTIO_RING_F_INDIRECT_DESC wouldn't apply to virtio-vbus.
The PIT uses a common state structure and common code for save/restore. This makes migration compatible.This would break things like migrating between userspace and kernel virtio (something that I support).Isn't device name put in the machine config, which presumably is send along as well?
Good question. I don't know the best way to resolve this.Maybe migration between devices isn't such a good idea. It's conceivable that vhost will require some state that isn't present in the userspace virtio-net. I think this requires some thought.
In this case, it's two separate implementations of the same device. I think it makes sense for them to be separate devices.Regards, Anthony LiguoriHmm, I see what you mean. But kernel virtio is harder. Unlike PIT/APIC, it is not a separate codepath. It still needs all of userspace virtio to support live migration and non-MSI guests. Really, it's the same device that switches between kernel and userspace modes on the fly. This will become clearer from code when I implement migration for vhost, but basically you switch to userspace when you start migration, and back to kernel if migration fails. You also switch to kernel when MSI is enabled and back to userspace when it is disabled.
Why bother switching to userspace for migration? Can't you just have get/set ioctls for the state?
Regards, Anthony Liguori
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