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

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

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 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.

        Joao

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