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From: Juergen Gross <jgross@suse.com>
To: Ankur Arora <ankur.a.arora@oracle.com>,
	Joao Martins <joao.m.martins@oracle.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, "Stefano Stabellini" <sstabellini@kernel.org>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH RFC 00/39] x86/KVM: Xen HVM guest support
Date: Wed, 10 Apr 2019 09:14:55 +0200	[thread overview]
Message-ID: <c488152c-b97f-c6ce-1f4e-72aff05d29dd@suse.com> (raw)
In-Reply-To: <8e2e1d56-3490-365c-e2de-c3fd262518ba@oracle.com>

On 10/04/2019 08:55, Ankur Arora wrote:
> On 2019-04-08 10:04 p.m., Juergen Gross wrote:
>> 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.
> Thanks this is clarifying.
> 
> So, essentially backend drivers with a xenhost_t handle, communicate
> with Xen low-level drivers etc using the same handle, however, if they
> communicate with frontend drivers for accessing the "real" world,
> they exclusively use standard mechanisms (Linux or hypercalls)?

This should be opaque to the backends. The xenhost_t handle should have
a pointer to a function vector for relevant grant-, event- and Xenstore-
related functions. Calls to such functions should be done via an inline
function with the xenhost_t handle being one parameter, that function
will then call the correct implementation.

> In this scenario L2 dom0 xen-netback and L2 dom0 xen-netfront should
> just be able to use Linux interfaces. But if L2 dom0 xenbus-backend
> needs to talk to L2 dom0 xenbus-frontend then do you see them layered
> or are they still exclusively talking via the standard mechanisms?

The distinction is made via the function vector in xenhost_t. So the
only change in backends needed is the introduction of xenhost_t.

Whether we want to introduce xenhost_t in frontends, too, is TBD.


Juergen

  reply	other threads:[~2019-04-10  7:15 UTC|newest]

Thread overview: 126+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-20 20:15 Joao Martins
2019-02-20 20:15 ` [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling Joao Martins
2019-02-22  1:30   ` Sean Christopherson
2019-02-22 11:47     ` Joao Martins
2019-02-22 12:51     ` Paolo Bonzini
2020-11-30 10:39       ` David Woodhouse
2020-11-30 11:03         ` Paolo Bonzini
2020-11-30 11:27           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if enabled Joao Martins
2019-02-21 18:29   ` Sean Christopherson
2019-02-21 20:56     ` Joao Martins
2019-02-22  0:30       ` Sean Christopherson
2019-02-22 12:50         ` Paolo Bonzini
2020-12-01  9:48   ` David Woodhouse
2020-12-01 11:19     ` David Woodhouse
2020-12-02 11:17       ` Joao Martins
2020-12-02 12:12         ` David Woodhouse
2020-12-02  5:19     ` Ankur Arora
2020-12-02  8:03       ` David Woodhouse
2020-12-02 18:20         ` Ankur Arora
2019-02-20 20:15 ` [PATCH RFC 03/39] KVM: x86/xen: register shared_info page Joao Martins
2020-12-01 13:07   ` David Woodhouse
2020-12-02  0:40     ` Ankur Arora
2020-12-02  1:26       ` David Woodhouse
2020-12-02  5:17         ` Ankur Arora
2020-12-02 10:50           ` Joao Martins
2020-12-02 10:44       ` Joao Martins
2020-12-02 12:20         ` David Woodhouse
2020-12-02 20:32           ` Ankur Arora
2020-12-03 10:16             ` David Woodhouse
2020-12-04 17:30               ` Sean Christopherson
2020-12-02 20:33         ` Ankur Arora
2020-12-12 12:07       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 04/39] KVM: x86/xen: setup pvclock updates Joao Martins
2019-02-20 20:15 ` [PATCH RFC 05/39] KVM: x86/xen: update wallclock region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 06/39] KVM: x86/xen: register vcpu info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 07/39] KVM: x86/xen: register vcpu time info region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 08/39] KVM: x86/xen: register steal clock Joao Martins
2019-02-20 20:15 ` [PATCH RFC 09/39] KVM: x86: declare Xen HVM guest capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 10/39] KVM: x86/xen: support upcall vector Joao Martins
2020-12-02 11:17   ` David Woodhouse
2020-12-02 13:12     ` Joao Martins
2020-12-02 16:47       ` David Woodhouse
2020-12-02 18:34         ` Joao Martins
2020-12-02 19:02           ` David Woodhouse
2020-12-02 20:12             ` Joao Martins
2020-12-02 20:37               ` David Woodhouse
2020-12-03  1:08             ` Ankur Arora
2020-12-08 16:08             ` David Woodhouse
2020-12-09  6:35               ` Ankur Arora
2020-12-09 10:27                 ` David Woodhouse
2020-12-09 10:51                   ` Joao Martins
2020-12-09 11:39                     ` David Woodhouse
2020-12-09 13:26                       ` Joao Martins
2020-12-09 15:41                         ` David Woodhouse
2020-12-09 16:12                           ` Joao Martins
2021-01-01 14:33           ` David Woodhouse
2021-01-05 12:11             ` Joao Martins
2021-01-05 13:23               ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd Joao Martins
2020-11-30  9:41   ` David Woodhouse
2020-11-30 12:17     ` Joao Martins
2020-11-30 12:55       ` David Woodhouse
2020-11-30 15:08         ` Joao Martins
2020-11-30 16:48           ` David Woodhouse
2020-11-30 17:15             ` Joao Martins
2020-11-30 18:01               ` David Woodhouse
2020-11-30 18:41                 ` Joao Martins
2020-11-30 19:04                   ` David Woodhouse
2020-11-30 19:25                     ` Joao Martins
2021-11-23 13:15           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 12/39] KVM: x86/xen: store virq when assigning evtchn Joao Martins
     [not found]   ` <b750291466f3c89e0a393e48079c087704b217a5.camel@amazon.co.uk>
2022-02-10 12:17     ` Joao Martins
2022-02-10 15:23       ` [EXTERNAL] " David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 13/39] KVM: x86/xen: handle PV timers oneshot mode Joao Martins
2019-02-20 20:15 ` [PATCH RFC 14/39] KVM: x86/xen: handle PV IPI vcpu yield Joao Martins
2019-02-20 20:15 ` [PATCH RFC 15/39] KVM: x86/xen: handle PV spinlocks slowpath Joao Martins
2022-02-08 12:36   ` David Woodhouse
2022-02-10 12:17     ` Joao Martins
2022-02-10 14:11       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 16/39] KVM: x86: declare Xen HVM evtchn offload capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 17/39] x86/xen: export vcpu_info and shared_info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 18/39] x86/xen: make hypercall_page generic Joao Martins
2019-02-20 20:15 ` [PATCH RFC 19/39] xen/xenbus: xenbus uninit support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 20/39] xen-blkback: module_exit support Joao Martins
2019-02-25 18:57   ` Konrad Rzeszutek Wilk
2019-02-26 11:20     ` Joao Martins
2019-02-20 20:15 ` [PATCH RFC 21/39] KVM: x86/xen: domid allocation Joao Martins
2019-02-20 20:15 ` [PATCH RFC 22/39] KVM: x86/xen: grant table init Joao Martins
2019-02-20 20:15 ` [PATCH RFC 23/39] KVM: x86/xen: grant table grow support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 24/39] KVM: x86/xen: backend hypercall support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 25/39] KVM: x86/xen: grant map support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 26/39] KVM: x86/xen: grant unmap support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 27/39] KVM: x86/xen: grant copy support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 28/39] KVM: x86/xen: interdomain evtchn support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 29/39] KVM: x86/xen: evtchn unmask support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 30/39] KVM: x86/xen: add additional evtchn ops Joao Martins
2019-02-20 20:16 ` [PATCH RFC 31/39] xen-shim: introduce shim domain driver Joao Martins
2019-02-20 20:16 ` [PATCH RFC 32/39] xen/balloon: xen_shim_domain() support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 33/39] xen/grant-table: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 34/39] xen/gntdev: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 35/39] xen/xenbus: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 36/39] drivers/xen: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 37/39] xen-netback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 38/39] xen-blkback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 39/39] KVM: x86: declare Xen HVM Dom0 capability Joao Martins
2019-02-20 21:09 ` [PATCH RFC 00/39] x86/KVM: Xen HVM guest support Paolo Bonzini
2019-02-21  0:29   ` Ankur Arora
2019-02-21 11:45   ` Joao Martins
2019-02-22 16:59     ` Paolo Bonzini
2019-03-12 17:14       ` Joao Martins
2019-04-08  6:44         ` Juergen Gross
2019-04-08 10:36           ` Joao Martins
2019-04-08 10:42             ` Juergen Gross
2019-04-08 17:31               ` Joao Martins
2019-04-09  0:35                 ` Stefano Stabellini
2019-04-10  5:50                   ` [Xen-devel] " Ankur Arora
2019-04-10 20:45                     ` Stefano Stabellini
2019-04-09  5:04                 ` Juergen Gross
2019-04-10  6:55                   ` Ankur Arora
2019-04-10  7:14                     ` Juergen Gross [this message]
2019-02-20 23:39 ` [Xen-devel] " Marek Marczykowski-Górecki
2019-02-21  0:31   ` Ankur Arora
2019-02-21  7:57   ` Juergen Gross
2019-02-21 12:00     ` Joao Martins
2019-02-21 11:55   ` Joao Martins

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