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

[Ankur Arora <ankur.a.arora@oracle.com>]

On 2019-04-08 5:35 p.m., Stefano Stabellini wrote:
> On Mon, 8 Apr 2019, 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 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.
> 
> If you are up for it, it would be great to write some documentation too.
> We are starting to have decent docs for some PV protocols, describing a
> specific PV interface, but we are lacking docs about the basic building
> blocks to bring up PV drivers in general. They would be extremely
Agreed. These would be useful.

> useful. Given that you have just done the work, you are in a great
> position to write those docs. Even bad English would be fine, I am sure
> somebody else could volunteer to clean-up the language. Anything would
> help :-)
Can't make any promises on this yet but I will see if I can convert
notes I made into something useful for the community.


Ankur

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