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

From: Juergen Gross
Date: Mon Apr 08 2019 - 06:43:05 EST

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 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. :-)

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.