From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-788-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Date: Wed, 10 Jul 2019 18:45:27 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20190710174527.GA27706@work-vm> References: <20190627192830.5285-1-dgilbert@redhat.com> <20190627192830.5285-3-dgilbert@redhat.com> <20190628120018.2bc8c387.cohuck@redhat.com> <20190628133614.GF2922@work-vm> <20190703150442.43b85b87.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190703150442.43b85b87.cohuck@redhat.com> Subject: [virtio-comment] Re: [virtio-dev] Re: [PATCH v5 2/5] pci: Define id field To: Cornelia Huck Cc: virtio-dev@lists.oasis-open.org, virtio-comment@lists.oasis-open.org, stefanha@redhat.com, vgoyal@redhat.com List-ID: * Cornelia Huck (cohuck@redhat.com) wrote: > On Fri, 28 Jun 2019 14:36:14 +0100 > "Dr. David Alan Gilbert" wrote: > > > * Cornelia Huck (cohuck@redhat.com) wrote: > > > On Thu, 27 Jun 2019 20:28:27 +0100 > > > "Dr. David Alan Gilbert (git)" wrote: > > > > > > > From: "Dr. David Alan Gilbert" > > > > > > > > For the virtio-fs device we require multiple large shared memory > > > > regions. Differentiate these by an 'id' field in the base capability. > > > > > > > > Signed-off-by: Dr. David Alan Gilbert > > > > --- > > > > content.tex | 8 +++++++- > > > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/content.tex b/content.tex > > > > index 6433226..41926c0 100644 > > > > --- a/content.tex > > > > +++ b/content.tex > > > > @@ -651,7 +651,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option > > > > u8 cap_len; /* Generic PCI field: capability length */ > > > > u8 cfg_type; /* Identifies the structure. */ > > > > u8 bar; /* Where to find it. */ > > > > - u8 padding[3]; /* Pad to full dword. */ > > > > + u8 id; /* Multiple capabilities of the same type */ > > > > + u8 padding[2]; /* Pad to full dword. */ > > > > le32 offset; /* Offset within bar. */ > > > > le32 length; /* Length of the structure, in bytes. */ > > > > }; > > > > @@ -716,6 +717,11 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option > > > > > > > > Any other value is reserved for future use. > > > > > > > > +\item[\field{id}] > > > > + Multiple capabilities of the same type can exist as long > > > > + as they each have a unique \field{id}. The specific > > > > > > The requirement for id is new, isn't it? Shouldn't it rather be > > > optional? > > > > Yes it is; this comes from moving it from the new structure we had it in > > the previous version, into the main _cap. > > > > Can I just say that only some devices use it, or do I need something > > more? Can I assume that 'id' is 0 for existing devices or is it > > undefined? > > Maybe something like the following? > > "Used by some device types to uniquely identify multiple capabilities > of a certain type. If the device type does not specify the meaning of > this field, its contents are undefined." > Thanks; slotted that in. > We should not make any assumptions here, I think. > > > > > > + meaning of the field is different for each device type. > > > > + > > > > \item[\field{offset}] > > > > indicates where the structure begins relative to the base address associated > > > > with the BAR. The alignment requirements of \field{offset} are indicated > > > > > > The current specification for cfg_type defines a kind of hierarchy for > > > capabilities of the same type (first one is preferred). We probably > > > need to be more explicit how id may interact with that (even if it is > > > device type specific). > > > > One way I was thinking was to say that the hierarchy now applies to > > pairs of {cfg_type, id} rather than just cfg_type; but that depends on > > the answer to the previous question to avoid breaking any existing > > hierarchies. > > If we make the meaning of id device type specific, we also need to make > its effects on the ordering device type specific. > > What about keeping the hierarchy, but specifying that a device type may > override it if it specifies usage of the id field? Each device type > using id would need to explicitly specify the ordering in that case. OK, I've gone with: The device MAY offer more than one structure of any type - this makes it possible for the device to expose multiple interfaces to drivers. The order of the capabilities in the capability list specifies the order of preference - suggested by the device. + suggested by the device. A device may specify that this ordering mechanism be + overridden by the use of the \field{id} field. \begin{note} -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. 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