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
