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Subject: Re: [virtio-comment] Re: [virtio] [PATCH] virtio: clarify feature negotiation
[sorry about resurrecting an undead thread; I'm trying to collect some editorial changes] On Wed, Jan 26 2022, Jean-Philippe Brucker <jean-philippe@linaro.org> wrote: > On Mon, Jan 24, 2022 at 04:42:35PM -0500, Michael S. Tsirkin wrote: >> So from that point of view, we are actually relaxing the requirements, >> and yes we'll need to carefully go over each instance of >> "offered". >> I thought I did, but now I did a quick grep again and I found instances >> in virtio-iommu.tex and virtio-gpio.tex >> Both of these are new, so I think we can just fix them to "negotiated". >> >> Jean-Philippe, Viresh, what do you think? Would it be a problem to >> replace offered with negotiated, and restrict iommu and gpio drivers to >> access config space after FEATURES_OK? > > No problem for iommu. The config space fields domain_range and input_range > are not useful to the driver before DRIVER_OK. Reading field bypass > earlier could be useful for debugging, but in that case going a few more > steps into device initialization wouldn't hurt. For domain_range and input_range, offered->negotiated looks like the right thing to do. However, I'm a bit unsure about bypass. We currently have: " An endpoint is in bypass mode if: \begin{itemize} \item the VIRTIO_IOMMU_F_BYPASS_CONFIG feature is offered and: \begin{itemize} \item config field \field{bypass} is 1 and the endpoint is not attached to a domain. This applies even if the driver does not accept the VIRTIO_IOMMU_F_BYPASS_CONFIG feature and the device initializes field \field{bypass} to 1. or \item the endpoint is attached to a domain with VIRTIO_IOMMU_ATTACH_F_BYPASS. \end{itemize} " The "This applies even if..." clause refers to "offered, but not accepted". I assume we want to do offered->negotiated even here, but wanted to double-check.
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