Re: custom virt-io support (in user-mode-linux)

[Anton Ivanov <anton.ivanov@kot-begemot.co.uk>]

On 22/05/2019 14:02, Johannes Berg wrote:
> Hi,
> 
> While my main interest is mostly in UML right now [1] I've CC'ed the
> qemu and virtualization lists because something similar might actually
> apply to other types of virtualization.
> 
> I'm thinking about adding virt-io support to UML, but the tricky part is
> that while I want to use the virt-io basics (because it's a nice
> interface from the 'inside'), I don't actually want the stock drivers
> that are part of the kernel now (like virtio-net etc.) but rather
> something that integrates with wifi (probably building on hwsim).
> 
> The 'inside' interfaces aren't really a problem - just have a specific
> device ID for this, and then write a normal virtio kernel driver for it.
> 
> The 'outside' interfaces are where my thinking breaks down right now.
> 
> Looking at lkl, the outside is just all implemented in lkl as code that
> gets linked to the library, so in UML terms it'd just be extra 'outside'
> code like the timer handling or other netdev stuff we have today.
> Looking at qemu, it's of course also implemented there, and then
> interfaces with the real network, console abstraction, etc.
> 
> However, like I said above, I really need something very custom and not
> likely to make it upstream to any project (because what point is that if
> you cannot connect to the rest of the environment I'm building), so I'm
> thinking that perhaps it should be possible to write an abstract
> 'outside' that lets you interact with it really from out-of-process?
> Perhaps through some kind of shared memory segment? I think that gets
> tricky with virt-io doing DMA (I think it does?) though, so that part
> would have to be implemented directly and not out-of-process?
> 
> But really that's why I'm asking - is there a better way than to just
> link the device-side virt-io code into the same binary (be it lkl lib,
> uml binary, qemu binary)?
> 
> Thanks,
> johannes
> 
> [1] Actually, I've considered using qemu, but it doesn't have
> virtualized time and doesn't seem to support TSC virtualization. I guess
> I could remove TSC from the guest CPU and add a virtualized HPET, but
> I've yet to convince myself this works - on UML I made virtual time as a
> prototype already:
> https://patchwork.ozlabs.org/patch/1095814/
> (though my real goal isn't to just skip time forward when the host goes
> idle, it's to sync with other simulated components)
> 
> 
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I have looked at using virtio semantics in UML in the past around the 
point when I wanted to make the recvmmsg/sendmmsg vector drivers common 
in UML and QEMU. It is certainly possible,

I went for the native approach at the end though.

-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/