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

[Paolo Bonzini]

On 21/02/19 12:45, Joao Martins wrote:
> On 2/20/19 9:09 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 guess Ankur already answered this; so just to stack this on top of his comment.
> 
> The xen_shim_domain() is only meant to handle the case where the backend
> has/can-have full access to guest memory [i.e. netback and blkback would work
> with similar assumptions as vhost?]. For the normal case, where a backend *in a
> guest* maps and unmaps other guest memory, this is not applicable and these
> changes don't affect that case.
> 
> IOW, the PV backend here sits on the hypervisor, and the hypercalls aren't
> actual hypercalls but rather invoking shim_hypercall(). The call chain would go
> more or less like:
> 
> <netback|blkback|scsiback>
>  gnttab_map_refs(map_ops, pages)
>    HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,...)
>      shim_hypercall()
>        shim_hcall_gntmap()
> 
> Our reasoning was that given we are already in KVM, why mapping a page if the
> user (i.e. the kernel PV backend) is himself? The lack of GNTMAP_host_map is how
> the shim determines its user doesn't want to map the page. Also, there's another
> issue where PV backends always need a struct page to reference the device
> inflight data as Ankur pointed out.

Ultimately it's up to the Xen people.  It does make their API uglier,
especially the in/out change for the parameter.  If you can at least
avoid that, it would alleviate my concerns quite a bit.

Paolo