Re: [Xen-devel] VIRTIO - compatibility with different virtualization solutions

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Hey,

On Thu, Feb 20, 2014 at 12:01:19PM +1030, Rusty Russell wrote:
> Anthony Liguori <anthony@xxxxxxxxxxxxx> writes:
> > On Tue, Feb 18, 2014 at 4:26 PM, Rusty Russell <rusty@xxxxxxxxxxx> wrote:
> >> Daniel Kiper <daniel.kiper@xxxxxxxxxx> writes:
> >>> Hi,
> >>>
> >>> Below you could find a summary of work in regards to VIRTIO compatibility 
> >>> with
> >>> different virtualization solutions. It was done mainly from Xen point of 
> >>> view
> >>> but results are quite generic and can be applied to wide spectrum
> >>> of virtualization platforms.
> >>
> >> Hi Daniel,
> >>
> >>         Sorry for the delayed response, I was pondering...  CC changed
> >> to virtio-dev.

Do not worry. It is not a problem. It is not easy issue.

> >> From a standard POV: It's possible to abstract out the where we use
> >> 'physical address' for 'address handle'.  It's also possible to define
> >> this per-platform (ie. Xen-PV vs everyone else).  This is sane, since
> >> Xen-PV is a distinct platform from x86.
> >
> > I'll go even further and say that "address handle" doesn't make sense too.
>
> I was trying to come up with a unique term, I wasn't trying to define
> semantics :)
>
> There are three debates here now: (1) what should the standard say, and

Yep.

> (2) how would Linux implement it,

It seems to me that we should think about other common OSes too.

> (3) should we use each platform's PCI IOMMU.

I do not want emulate any hardware. It seems to me that we should think about
something which fits best in VIRTIO environment. DMA API with relevant backends
looks promising but I have also some worries about performance. Additionally,
it is Linux Kernel specific stuff so maybe we should invent something more 
generic
which will fit well in other guest OSes too.

[...]

> It's a fundamental assumption of virtio that the host can access all of
> guest memory.  That's paravert, not a hack.

Why? What if guests would like to limit access to their memory? I think
that it will happen sooner or later. Additionally, I think that your
assumption is not hypervisor agnostic which limits implementation of
VIRTIO spec. At least for Xen your idea will make difficulties and
probably prevent VRITIO implementation.

Daniel

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