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


On Wed, Jun 19, 2019 at 06:13:17PM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (mst@redhat.com) wrote:
> > On Mon, Jun 17, 2019 at 02:36:43PM -0400, Michael S. Tsirkin 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?
> > > 
> > > > +};
> > > > +\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.
> > 
> > And maybe we should set a bit in some other reserved byte
> > so people can generally use 64 bit offsets/lengths?
> 
> Is it actually any cleaner?
> 
> Dave

I am just saying this: let's assume some other config type will also
want to use 64 bit offsets down the road. If we do it will be nice
to be able, down the line, to write
if (bit set)
	64 bit
else
	32 bit

without special-casing this config type.


> I don't know - where would you put that bit?

options include cfg type and bar.
I think I prefer bar since their number is definitely limited by pci
spec.

> > > 
> > > > +
> > > > +While most capabilities can not be repeated,
> > > 
> > > I'd drop this part.
> > > 
> > > > 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.
> > > 
> > > >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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