[virtio-comment] Re: [virtio-dev] [PATCH v4 2/3] shared memory: Define PCI capability

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

On Wed, Jun 19, 2019 at 06:10:45PM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
> > On Wed, Mar 20, 2019 at 12:07:39PM +0000, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > Define the PCI capability used for enumerating shared memory regions.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> > > ---
> > >  content.tex | 37 +++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 37 insertions(+)
> > > 
> > > diff --git a/content.tex b/content.tex
> > > index 8cd1a38..ddbf949 100644
> > > --- a/content.tex
> > > +++ b/content.tex
> > > @@ -688,6 +688,8 @@ The fields are interpreted as follows:
> > >  #define VIRTIO_PCI_CAP_DEVICE_CFG        4
> > >  /* PCI configuration access */
> > >  #define VIRTIO_PCI_CAP_PCI_CFG           5
> > > +/* Shared memory region */
> > > +#define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8
> > >  \end{lstlisting}
> > >  
> > >          Any other value is reserved for future use.
> > > @@ -1052,6 +1054,41 @@ any device type which has a device-specific configuration.
> > >  
> > >  The \field{offset} for the device-specific configuration MUST be 4-byte aligned.
> > >  
> > > +\subsubsection{Shared memory capability}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Shared memory capability}
> > > +
> > > +Shared memory regions \ref{sec:Basic Facilities of a Virtio
> > > +Device / Shared Memory Regions} are enumerated on the PCI transport
> > > +as a sequence of VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capabilities, one per region.
> > > +
> > > +The capability is immediately followed by an additional field like so:
> > > +
> > > +\begin{lstlisting}
> > > +struct virtio_pci_shm_cap {
> > > +        struct virtio_pci_cap cap;
> > > +        u32 offset_hi;
> > > +        u32 length_hi;
> > > +        u8  id;
> > 
> > Pls pad, pci capabilities must be a multiple of 4 bytes.
> > BTW virtio_pci_cap has 3 bytes of padding.
> > We can stick the id there maybe?
> 
> Do you mean changing the def of virtio_pci_cap to: ?
> struct virtio_pci_cap {
>         __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. */
>         __u8 bar;               /* Where to find it. */
>         __u8 id;                /* Unique ID for this region */
>         __u8 padding[2];        /* Pad to full dword. */
>         __le32 offset;          /* Offset within bar. */
>         __le32 length;          /* Length of the structure, in bytes. */
> };

Something like this, yes.

> > > +};
> > > +\end{lstlisting}
> > > +
> > > +Given that the \field{cap.length} and \field{cap.offset} fields
> > > +are only 32 bit, the additional \field{offset_hi} and \field {length_hi}
> > > +fields provide the most significant 32 bits of a total 64 bit offset and
> > > +length within the bar specified by \field{cap.bar}.  \field{id}
> > > +uniquely identifies the given shared memory region using a device
> > > +specific identifier.
> > 
> > I didn't realize we will ever need so much space.  I think I prefer
> > moving offset_hi/length_hi to a general virtio_pci_cap64 that others can
> > reuse.
> 
> Would we then just rename virtio_pci_shm_cap to virtio_pci_cap64 after
> moving the id to virito_pci_cap?

Right except we then need to document that only shm uses cap64, others
use cap. And I guess for forward compatibility set a bit somewhere - within bar maybe?

Overall I'm no longer sure it's a good idea. you decide.

> > 
> > 
> > > +
> > > +While most capabilities can not be repeated,
> > 
> > I'd drop this part.
> 
> Done.
> 
> > > multiple
> > > +VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capabilities are allowed as long
> > > +as the \field{id} is unique.
> > > +
> > > +\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 offsets and length
> > > +fields 
> > 
> > let's do \field{cap.length} \field{cap.offset} etc.
> 
> I've done that as:
> 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.
> 
> is that right, but the base and hi are in separate structures.


ok

> > >MUST be contained within the declared bar.
> > > +
> > > +The \field{id} MUST be unique for any one device instance.
> > > +
> > >  \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
> > > -- 
> > > 2.20.1
> > > 
> > > 
> > > ---------------------------------------------------------------------
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> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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