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

[Juergen Gross]

On 08/04/2019 19:31, Joao Martins wrote:
> On 4/8/19 11:42 AM, Juergen Gross wrote:
>> On 08/04/2019 12:36, Joao Martins wrote:
>>> On 4/8/19 7:44 AM, Juergen Gross wrote:
>>>> On 12/03/2019 18:14, Joao Martins wrote:
>>>>> On 2/22/19 4:59 PM, Paolo Bonzini wrote:
>>>>>> 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.
>>>>>
>>>>> In my view, we have two options overall:
>>>>>
>>>>> 1) Make it explicit, the changes the PV drivers we have to make in
>>>>> order to support xen_shim_domain(). This could mean e.g. a) add a callback
>>>>> argument to gnttab_map_refs() that is invoked for every page that gets looked up
>>>>> successfully, and inside this callback the PV driver may update it's tracking
>>>>> page. Here we no longer have this in/out parameter in gnttab_map_refs, and all
>>>>> shim_domain specific bits would be a little more abstracted from Xen PV
>>>>> backends. See netback example below the scissors mark. Or b) have sort of a
>>>>> translate_gref() and put_gref() API that Xen PV drivers use which make it even
>>>>> more explicit that there's no grant ops involved. The latter is more invasive.
>>>>>
>>>>> 2) The second option is to support guest grant mapping/unmapping [*] to allow
>>>>> hosting PV backends inside the guest. This would remove the Xen changes in this
>>>>> series completely. But it would require another guest being used
>>>>> as netback/blkback/xenstored, and less performance than 1) (though, in theory,
>>>>> it would be equivalent to what does Xen with grants/events). The only changes in
>>>>> Linux Xen code is adding xenstored domain support, but that is useful on its own
>>>>> outside the scope of this work.
>>>>>
>>>>> I think there's value on both; 1) is probably more familiar for KVM users
>>>>> perhaps (as it is similar to what vhost does?) while  2) equates to implementing
>>>>> Xen disagregation capabilities in KVM.
>>>>>
>>>>> Thoughts? Xen maintainers what's your take on this?
>>>>
>>>> What I'd like best would be a new handle (e.g. xenhost_t *) used as an
>>>> abstraction layer for this kind of stuff. It should be passed to the
>>>> backends and those would pass it on to low-level Xen drivers (xenbus,
>>>> event channels, grant table, ...).
>>>>
>>> So if IIRC backends would use the xenhost layer to access grants or frames
>>> referenced by grants, and that would grok into some of this. IOW, you would have
>>> two implementors of xenhost: one for nested remote/local events+grants and
>>> another for this "shim domain" ?
>>
>> As I'd need that for nested Xen I guess that would make it 3 variants.
>> Probably the xen-shim variant would need more hooks, but that should be
>> no problem.
>>
> I probably messed up in the short description but "nested remote/local
> events+grants" was referring to nested Xen (FWIW remote meant L0 and local L1).
> So maybe only 2 variants are needed?

I need one xenhost variant for the "normal" case as today: talking to
the single hypervisor (or in nested case: to the L1 hypervisor).

Then I need a variant for the nested case talking to L0 hypervisor.

And you need a variant talking to xen-shim.

The first two variants can be active in the same system in case of
nested Xen: the backends of L2 dom0 are talking to L1 hypervisor,
while its frontends are talking with L0 hypervisor.

> 
>>>> I was planning to do that (the xenhost_t * stuff) soon in order to add
>>>> support for nested Xen using PV devices (you need two Xenstores for that
>>>> as the nested dom0 is acting as Xen backend server, while using PV
>>>> frontends for accessing the "real" world outside).
>>>>
>>>> The xenhost_t should be used for:
>>>>
>>>> - accessing Xenstore
>>>> - issuing and receiving events
>>>> - doing hypercalls
>>>> - grant table operations
>>>>
>>>
>>> In the text above, I sort of suggested a slice of this on 1.b) with a
>>> translate_gref() and put_gref() API -- to get the page from a gref. This was
>>> because of the flags|host_addr hurdle we depicted above wrt to using using grant
>>> maps/unmaps. You think some of the xenhost layer would be ammenable to support
>>> this case?
>>
>> I think so, yes.
>>
>>>
>>>> So exactly the kind of stuff you want to do, too.
>>>>
>>> Cool idea!
>>
>> In the end you might make my life easier for nested Xen. :-)
>>
> Hehe :)
> 
>> Do you want to have a try with that idea or should I do that? I might be
>> able to start working on that in about a month.
>>
> Ankur (CC'ed) will give a shot at it, and should start a new thread on this
> xenhost abstraction layer.

Great, looking forward to it!


Juergen