Re: [Xen-devel] [RFC PATCH 00/16] xenhost support

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

On 6/7/19 3:51 PM, Juergen Gross wrote:
> On 09.05.19 19:25, Ankur Arora wrote:
>> Hi all,
>>
>> This is an RFC for xenhost support, outlined here by Juergen here:
>> https://lkml.org/lkml/2019/4/8/67.
> 
> First: thanks for all the effort you've put into this series!
> 
>> The high level idea is to provide an abstraction of the Xen
>> communication interface, as a xenhost_t.
>>
>> xenhost_t expose ops for communication between the guest and Xen
>> (hypercall, cpuid, shared_info/vcpu_info, evtchn, grant-table and on top
>> of those, xenbus, ballooning), and these can differ based on the kind
>> of underlying Xen: regular, local, and nested.
> 
> I'm not sure we need to abstract away hypercalls and cpuid. I believe in
> case of nested Xen all contacts to the L0 hypervisor should be done via
> the L1 hypervisor. So we might need to issue some kind of passthrough
> hypercall when e.g. granting a page to L0 dom0, but this should be
> handled via the grant abstraction (events should be similar).
> 
Just to be clear: By "kind of passthrough hypercall" you mean (e.g. for every
access/modify of grant table frames) you would proxy hypercall to L0 Xen via L1 
Xen?

> So IMO we should drop patches 2-5.
> 
>> (Since this abstraction is largely about guest -- xenhost communication,
>> no ops are needed for timer, clock, sched, memory (MMU, P2M), VCPU mgmt.
>> etc.)
>>
>> Xenhost use-cases:
>>
>> Regular-Xen: the standard Xen interface presented to a guest,
>> specifically for comunication between Lx-guest and Lx-Xen.
>>
>> Local-Xen: a Xen like interface which runs in the same address space as
>> the guest (dom0). This, can act as the default xenhost.
>>
>> The major ways it differs from a regular Xen interface is in presenting
>> a different hypercall interface (call instead of a syscall/vmcall), and
>> in an inability to do grant-mappings: since local-Xen exists in the same
>> address space as Xen, there's no way for it to cheaply change the
>> physical page that a GFN maps to (assuming no P2M tables.)
>>
>> Nested-Xen: this channel is to Xen, one level removed: from L1-guest to
>> L0-Xen. The use case is that we want L0-dom0-backends to talk to
>> L1-dom0-frontend drivers which can then present PV devices which can
>> in-turn be used by the L1-dom0-backend drivers as raw underlying devices.
>> The interfaces themselves, broadly remain similar.
>>
>> Note: L0-Xen, L1-Xen represent Xen running at that nesting level
>> and L0-guest, L1-guest represent guests that are children of Xen
>> at that nesting level. Lx, represents any level.
>>
>> Patches 1-7,
>>    "x86/xen: add xenhost_t interface"
>>    "x86/xen: cpuid support in xenhost_t"
>>    "x86/xen: make hypercall_page generic"
>>    "x86/xen: hypercall support for xenhost_t"
>>    "x86/xen: add feature support in xenhost_t"
>>    "x86/xen: add shared_info support to xenhost_t"
>>    "x86/xen: make vcpu_info part of xenhost_t"
>> abstract out interfaces that setup hypercalls/cpuid/shared_info/vcpu_info 
>> etc.
>>
>> Patch 8, "x86/xen: irq/upcall handling with multiple xenhosts"
>> sets up the upcall and pv_irq ops based on vcpu_info.
>>
>> Patch 9, "xen/evtchn: support evtchn in xenhost_t" adds xenhost based
>> evtchn support for evtchn_2l.
>>
>> Patches 10 and 16, "xen/balloon: support ballooning in xenhost_t" and
>> "xen/grant-table: host_addr fixup in mapping on xenhost_r0"
>> implement support from GNTTABOP_map_grant_ref for xenhosts of type
>> xenhost_r0 (xenhost local.)
>>
>> Patch 12, "xen/xenbus: support xenbus frontend/backend with xenhost_t"
>> makes xenbus so that both its frontend and backend can be bootstrapped
>> separately via separate xenhosts.
>>
>> Remaining patches, 11, 13, 14, 15:
>>    "xen/grant-table: make grant-table xenhost aware"
>>    "drivers/xen: gnttab, evtchn, xenbus API changes"
>>    "xen/blk: gnttab, evtchn, xenbus API changes"
>>    "xen/net: gnttab, evtchn, xenbus API changes"
>> are mostly mechanical changes for APIs that now take xenhost_t *
>> as parameter.
>>
>> The code itself is RFC quality, and is mostly meant to get feedback before
>> proceeding further. Also note that the FIFO logic and some Xen drivers
>> (input, pciback, scsi etc) are mostly unchanged, so will not build.
>>
>>
>> Please take a look.
> 
> 
> Juergen
> 

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