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Subject: Re: [PATCH] Reserve more feature bits for device type usage
On Wed, Feb 02 2022, Max Gurtovoy <mgurtovoy@nvidia.com> wrote: > On 1/14/2022 1:12 PM, Cornelia Huck wrote: >> Feature bits 41 and above are noted as being reserved for future >> extensions. However, the net device has been using bits in that space >> for some time now, as it already used up the device type specific >> range up to 23. >> >> To avoid problems in the future, let's designate bits 50 to 127 to >> device type specific usage (which accommodates current usage by the >> net driver, and gives breathing room for future type specific bits), >> and declare bits 41 to 49 and bits 128 and above to be reserved for >> future extensions (which gives us some time before bit numbers move >> beyond 63, which would need some changes in existing device and driver >> implementations.) >> >> Reported-by: Max Gurtovoy <mgurtovoy@nvidia.com> >> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/131 >> Signed-off-by: Cornelia Huck <cohuck@redhat.com> >> --- >> content.tex | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/content.tex b/content.tex >> index 32de6685c50b..c6f116c7aa39 100644 >> --- a/content.tex >> +++ b/content.tex >> @@ -97,12 +97,12 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B >> Feature bits are allocated as follows: >> >> \begin{description} >> -\item[0 to 23] Feature bits for the specific device type >> +\item[0 to 23, and 50 to 127] Feature bits for the specific device type >> >> \item[24 to 40] Feature bits reserved for extensions to the queue and >> feature negotiation mechanisms >> >> -\item[41 and above] Feature bits reserved for future extensions. >> +\item[41 to 49, and 128 and above] Feature bits reserved for future extensions. >> \end{description} >> >> \begin{note} > > Cornelia, > > I've noticed that Legacy net device has VIRTIO_NET_F_GUEST_RSC4 (41) and > VIRTIO_NET_F_GUEST_RSC6 (42). > > Do we need to reserved them too or keep the fix as-is ? Eww. I think those bits shouldn't have been hiding in the "legacy" section, as their higher number indicates that they are not for a legacy device in the spec sense. They have been introduced in https://github.com/oasis-tcs/virtio-spec/issues/21 in 2018, I'm wondering how old those Windows drivers referred to in there are. IOW, would a new feature bit 41/42 break things for currently used Windows drivers if it showed up on a virtio-net device? If we still care about those drivers, we should reserve bits 41 and 42 as do-not-use, I guess.
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