linux-kernel - Re: [RFC PATCH 1/1] virtio: write back features before verify

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Date:   Sat, 2 Oct 2021 14:13:37 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, markver@...ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

On Fri, Oct 01, 2021 at 05:18:46PM +0200, Cornelia Huck wrote:
> I'd say we need a hack here so that we assume little-endian config space
> if VERSION_1 has been offered; if your patch here works, I assume QEMU
> does what we expect (assmuming little-endian as well.) I'm mostly
> wondering what happens if you use a different VMM; can we expect it to
> work similar to QEMU?

Hard to say of course ... hopefully other VMMs are actually
implementing the spec. E.g. IIUC rust vmm is modern only.


> Even if it helps for s390, we should double-check
> what happens for other architectures.
> 
> >
> >> 
> >> Anyone have any better suggestions?
> >> 
> >
> > There is the conditional compile, as an option but I would not say it is
> > better.
> 
> Yes, I agree.
> 
> Anyone else have an idea? This is a nasty regression; we could revert the
> patch, which would remove the symptoms and give us some time, but that
> doesn't really feel right, I'd do that only as a last resort.

Well we have Halil's hack (except I would limit it
to only apply to BE, only do devices with validate,
and only in modern mode), and we will fix QEMU to be spec compliant.
Between these why do we need any conditional compiles?

-- 
MST