[virtio] Re: [virtio-comment] [PATCH] content: add vendor specific cfg type

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 27, 2019 at 03:54:17PM +0100, Christophe de Dinechin wrote:
> 
> 
> > On 28 Oct 2019, at 11:55, Michael S. Tsirkin <mst@redhat.com> wrote:
> > 
> > Vendors might want to add their own capability
> > in the PCI capability list. However, Virtio already
> > uses the vendor specific capability ID (0x09)
> > for its own purposes.
> > 
> > Provide a structure for vendor specific extensions.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > content.tex | 80 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 80 insertions(+)
> > 
> > diff --git a/content.tex b/content.tex
> > index b1ea9b9..8a79b03 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -691,6 +691,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
> > #define VIRTIO_PCI_CAP_PCI_CFG           5
> > /* Shared memory region */
> > #define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8
> > +/* Vendor-specific data */
> > +#define VIRTIO_PCI_CAP_VENDOR_CFG        9
> > \end{lstlisting}
> > 
> >         Any other value is reserved for future use.
> > @@ -1099,6 +1101,84 @@ \subsubsection{Shared memory capability}\label{sec:Virtio Transport Options / Vi
> > 
> > The \field{cap.id} MUST be unique for any one device instance.
> > 
> > +\devicenormative{\paragraph}{Device-specific configuration}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Shared memory capability}
> > +
> > +The region defined by the combination of the \field {cap.offset},
> > +\field {cap.offset_hi}, and \field {cap.length}, \field
> > +{cap.length_hi} fields MUST be contained within the declared bar.
> 
> Uppercase BAR?

\field{bar} I think.

But really this is from "shared memory: Define PCI capability"
that I somehow failed to apply properly :(

> > +
> > +The \field{cap.id} MUST be unique for any one device instance.
> > +
> > +\subsubsection{Vendor data capability}\label{sec:Virtio
> > +Transport Options / Virtio Over PCI Bus / PCI Device Layout /
> > +Vendor data capability}
> > +
> > +The optional Vendor data capability allows the device to present
> > +vendor-specific data to the driver, without
> > +conflicts, for debugging and/or reporting purposes,
> > +and without conflicting with standard functionality.
> > +
> > +This capability augments but does not replace the standard
> > +subsystem ID and subsystem vendor ID fields
> > +(offsets 0x2C and 0x2E in the PCI configuration space header)
> > +as specified by \hyperref[intro:PCI]{[PCI]}.
> > +
> > +Vendor data capability is enumerated on the PCI transport
> > +as a VIRTIO_PCI_CAP_VENDOR_CFG capability.
> > +
> > +The capability has the following structure:
> > +\begin{lstlisting}
> > +struct virtio_pci_vndr_data {
> > +        u8 cap_vndr;    /* Generic PCI field: PCI_CAP_ID_VNDR */
> > +        u8 cap_next;    /* Generic PCI field: next ptr. */
> > +        u8 cap_len;     /* Generic PCI field: capability length */
> > +        u8 cfg_type;    /* Identifies the structure. */
> > +        u16 vendor_id;  /* Identifies the vendor-specific format. */
> > +	/* For Vendor Definition */
> > +	/* Pads structure to a multiple of 4 bytes */
> > +	/* Reads must not have side effects */
> > +};
> > +\end{lstlisting}
> > +
> > +Where \field{vendor_id} identifies the PCI-SIG assigned Vendor ID
> > +as specified by \hyperref[intro:PCI]{[PCI]}.
> > +
> > +Note that the capability size is required to be a multiple of 4.
> > +
> > +To make it safe for a generic driver to access the capability,
> > +reads from this capability MUST NOT have any side effects.
> 
> I think the intent is that this includes the capability payload, not
> just the generic fields. Worth spelling out?
> 
> > +
> > +\devicenormative{\subsection}{Vendor data capability}{Virtio
> > +Transport Options / Virtio Over PCI Bus / PCI Device Layout /
> > +Vendor data capability}
> > +
> > +The device SHOULD present the PCI subsystem vendor ID matching
> > +the device vendor, at offset 0x2C in its PCI configuration space
> > +header.
> > +
> > +Devices CAN present \field{vendor_id} that does not match
> > +either the PCI Vendor ID or the PCI Subsystem Vendor ID.
> > +
> > +Devices CAN present multiple Vendor data capabilities with
> > +either different or identical \field{vendor_id} values.
> > +
> > +The value \field{vendor_id} MUST NOT equal 0x1AF4.
> > +
> > +The size of the Vendor data capability MUST be a multiple of 4 bytes.
> > +
> > +Reads of the Vendor data capability by the driver MUST NOT have any
> > +side effects.
> > +
> > +\drivernormative{\subsection}{Vendor data capability}{Virtio
> > +Transport Options / Virtio Over PCI Bus / PCI Device Layout /
> > +Vendor data capability}
> > +
> > +The driver SHOULD NOT use the Vendor data capability except
> > +for debugging and reporting purposes.
> 
> What is the rationale for this restriction?
> 
> For example, would it be OK for a vendor to expose the presence
> of a particular firmware bugfix, so that the driver could disable
> some bug workaround? You seem to hint that this is the intent
> in a later response, but on initial reading, it was not clear to me
> that such usage would fit under “debugging” or “reporting”.
> 
> (I’m quite new here, so feel free to ignore if it’s clear to everybody else)
> 
> 
> > +
> > +The driver MUST qualify the \field{vendor_id} before
> > +interpreting or writing into the Vendor data capability.
> > +
> > \subsubsection{PCI configuration access capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / PCI configuration access capability}
> > 
> > The VIRTIO_PCI_CAP_PCI_CFG capability
> > -- 
> > MST
> > 
> > 
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> > OASIS Virtual I/O Device (VIRTIO) TC.
> > 
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