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
