Re: [PATCH RFC 00/39] x86/KVM: Xen HVM guest support

[Ankur Arora]

On 2/20/19 1:09 PM, Paolo Bonzini wrote:
> On 20/02/19 21:15, Joao Martins wrote:
> >   2. PV Driver support (patches 17 - 39)
> >
> >   We start by redirecting hypercalls from the backend to routines
> >   which emulate the behaviour that PV backends expect i.e. grant
> >   table and interdomain events. Next, we add support for late
> >   initialization of xenbus, followed by implementing
> >   frontend/backend communication mechanisms (i.e. grant tables and
> >   interdomain event channels). Finally, introduce xen-shim.ko,
> >   which will setup a limited Xen environment. This uses the added
> >   functionality of Xen specific shared memory (grant tables) and
> >   notifications (event channels).
>
> I am a bit worried by the last patches, they seem really brittle and
> prone to breakage.  I don't know Xen well enough to understand if the
> lack of support for GNTMAP_host_map is fixable, but if not, you have to
> define a completely different hypercall.
I assume you are aware of most of this, but just in case, here's the
flow when a backend driver wants to map a grant-reference in the
host: it allocates an empty struct page (via ballooning) and does a
map_grant_ref(GNTMAP_host_map) hypercall. In response, Xen validates the
grant-reference and maps it onto the address associated with the struct
page.
After this, from the POV of the underlying network/block drivers, these
struct pages can be used as just regular pages.

To support this in a KVM environment, where AFAICS no remapping of pages
is possible, the idea was to make minimal changes to the backend drivers
such that map_grant_ref() could just return the PFN from which the
backend could derive the struct page.

To ensure that backends -- when running in this environment -- have been
modified to deal with these new semantics, our map_grant_ref()
implementation explicitly disallows the GNTMAP_host_map flag.

Now if I'm reading you right, you would prefer something more
straightforward -- perhaps similar semantics but a new flag that
makes this behaviour explicit?

> Of course, tests are missing.  You should use the
> tools/testing/selftests/kvm/ framework, and ideally each patch should
> come with coverage for the newly-added code.
Agreed.

Thanks
Ankur

> Thanks,
>
> Paolo