linux-kernel - Re: [PATCH 0/2] virtio-balloon: make it spec compliant

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Message-ID: <77ba077a-a7a0-49b0-b14a-954cb24901e6@redhat.com>
Date: Fri, 5 Jul 2024 13:00:50 +0200
From: David Hildenbrand <david@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org,
 Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/2] virtio-balloon: make it spec compliant

On 05.07.24 12:19, Michael S. Tsirkin wrote:
> On Fri, Jul 05, 2024 at 12:15:30PM +0200, David Hildenbrand wrote:
>> On 05.07.24 12:08, Michael S. Tsirkin wrote:
>>> Currently, if VIRTIO_BALLOON_F_FREE_PAGE_HINT is off but
>>> VIRTIO_BALLOON_F_REPORTING is on, then the reporting vq
>>> gets number 3 while spec says it's number 4.
>>> It happens to work because the qemu virtio pci driver
>>> is *also* out of spec.
>>
>> I have to ask the obvious: maybe the spec is wrong and we have to refine
>> that?
> 
> Well having vq function shift depending on features is certainly
> messy ...

Right, but that's how all of this started from the beginning.

> How do we know no one implemented the spec as written though?

I understand that concern, IIUC it would imply that:

a) In case of a hypervisor, we never ran with a Linux guest
b) In case of a guest, we never ran under QEMU

It's certainly possible, although I would assume that most other 
implementation candidates (e.g., cloud-hypervisor) would have complained 
by now about Linux issues.

What's your experience: if someone would actually implement it according 
to the spec, would they watch out on the virtio mailing lists for 
changes (or even be able to vote) and would be able to comment that 
adjusting the spec to the real first implementation is wrong?

-- 
Cheers,

David / dhildenb