linux-kernel - Re: IOTLB support for vhost/vsock breaks crosvm on Android

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Date:   Sun, 7 Aug 2022 09:27:03 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will@...nel.org>, stefanha@...hat.com,
        jasowang@...hat.com, ascull@...gle.com, maz@...nel.org,
        keirf@...gle.com, jiyong@...gle.com, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Stefano Garzarella <sgarzare@...hat.com>
Subject: Re: IOTLB support for vhost/vsock breaks crosvm on Android

On Fri, Aug 05, 2022 at 03:57:08PM -0700, Linus Torvalds wrote:
> And hey, it's possible that the bit encoding is *so* incestuous that
> it's really hard to split it into two. But it really sounds to me like
> somebody mindlessly re-used a feature bit for a *completely* different
> thing. Why?
> 
> Why have feature bits at all, when you then re-use the same bit for
> two different features? It kind of seems to defeat the whole purpose.

What can I say? Hindsight is 20/20. The two things are
*related* in that IOTLB in vhost is a way for userspace
(the platform) to limit device access to guest memory.
So we reused the feature bits (it's not the only one,
just the one we changed most recently).
It bothered me a bit but everyone seemed happy and
was able to refer to virtio spec for documentation so there
was less documentation to write for Linux.

It's not that it's hard to split it generally, it's just that
it's been there like this for a while so it's hard to change
now - we need to find a way that does not break existing userspace.

-- 
MST