Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Tim Deegan <tim@xen.org>, Ian Jackson <ian.jackson@eu.citrix.com>,
	Eslam Elnikety <elnikety@amazon.com>,
	Julien Grall <julien.grall@arm.com>,
	Anthony PERARD <anthony.perard@citrix.com>,
	xen-devel@lists.xenproject.org,
	David Woodhouse <dwmw@amazon.co.uk>
Subject: Re: [Xen-devel] [PATCH] evtchn: make support for different ABIs tunable
Date: Wed, 7 Aug 2019 16:00:43 +0100	[thread overview]
Message-ID: <83a583e2-f4f2-8519-ce58-b200987dcab5@citrix.com> (raw)
In-Reply-To: <d0c34b24-697e-fbc1-3d7d-741e0f37c768@suse.com>

On 07/08/2019 15:30, Jan Beulich wrote:
> On 07.08.2019 15:41, Andrew Cooper wrote:
>> On 07/08/2019 12:20, Eslam Elnikety wrote:
>>> diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
>>> index 19486d5e32..654b4fdd22 100644
>>> --- a/xen/include/public/domctl.h
>>> +++ b/xen/include/public/domctl.h
>>> @@ -64,6 +64,9 @@ struct xen_domctl_createdomain {
>>>    /* Is this a xenstore domain? */
>>>   #define _XEN_DOMCTL_CDF_xs_domain     4
>>>   #define XEN_DOMCTL_CDF_xs_domain      (1U<<_XEN_DOMCTL_CDF_xs_domain)
>>> + /* Disable FIFO event channels? */
>>> +#define _XEN_DOMCTL_CDF_disable_fifo  5
>>> +#define XEN_DOMCTL_CDF_disable_fifo  
>>> (1U<<_XEN_DOMCTL_CDF_disable_fifo)
>>>       uint32_t flags;
>>
>> On the subject of the the patch itself, I think this is broadly the
>> right principle, but wants to be expressed differently.
>>
>> First, you'll want to rebase onto a very recent master, and specifically
>> over c/s d8f2490561eb which has changed how this field is handled in
>> Xen.
>>
>> Furthermore, if there is this problem for event channels, then there is
>> almost certainly a related problem for grant tables.
>>
>> The control in Xen should be expressed in a positive form, or the logic
>> will become a tangle.  It should be a bit permitting the use of the FIFO
>> ABI, rather than a bit saying "oh actually, you can't use that".
>>
>> That said, it might be easier to declare FIFO to be "event channel v2",
>> and specify max_{grant,evntchn}_ver instead.
>
> I'm not sure assuming linear (or actually any) ordering between
> variants is a good thing. Yes, right now we only have gnttab
> v1 and v2 and evtchn 2l and fifo, which could be considered v1
> and v2 as you suggest. However, assuming a 3rd variant surfaces,
> why would it be that one has to expose v2 just to make v3
> usable? In particular gnttab v2 has various issues (which is why
> you introduced a way to disable its use in the first place), yet
> I'd hope we'd end up with a less quirky v3 if one ever becomes
> necessary. And in turn I'd hope we could hide v2 from any v3
> users.
>
> IOW I think a bitmap to permit use of "advanced" versions is
> more future proof. (As a side note, I don't think we want to
> introduce a disable for the respective v1 interfaces.)

We absolutely do want a way to turn everything off.

The inability to turn the Xen extensions off for HVM guests (even if
only for debugging purposes) is a severely short sighted decision.

It is also a feature which has been requested repeatedly by users in the
past, and I am very deliberately building a way to do this into the
CPUID work.

However, it is an unreasonable request to bundle into this bugfix, hence
why I didn't suggest it.

Now I think about it, things like available ABIs really should be in the
Xen hypervisor CPUID leaves, but again, that ship sailed a decade ago.

~Andrew

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