[virtio-dev] Re: [PATCH V2 1/2] virtio-pci: introduce virtio structure PCI Extended Capability

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jan 14, 2022 at 11:23:00AM +0800, Jason Wang wrote:
> 
> 在 2022/1/13 下午6:19, Stefan Hajnoczi 写道:
> > On Thu, Jan 13, 2022 at 08:55:18AM +0800, Jason Wang wrote:
> > > On Wed, Jan 12, 2022 at 6:10 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > > On Wed, Jan 12, 2022 at 01:57:54PM +0800, Jason Wang wrote:
> > > > > We're already out of the configuration space if there's a device that
> > > > > supports all kinds of the virtio structure via PCI capability. This
> > > > > prevents us from adding new capabilities in the future.
> > > > > 
> > > > > So the patch adds the support for virtio structure via PCI Extended
> > > > > Capability via the vendor specific extended capability.
> > > > > 
> > > > > Only MMIO bar is allowed now.
> > > > > 
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > > ---
> > > > >   content.tex | 127 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > >   1 file changed, 127 insertions(+)
> > > > > 
> > > > > diff --git a/content.tex b/content.tex
> > > > > index cf20570..00a75f2 100644
> > > > > --- a/content.tex
> > > > > +++ b/content.tex
> > > > > @@ -1476,6 +1476,129 @@ \subsubsection{Non-transitional Device With Legacy Driver: A Note
> > > > >      of BAR0 by presenting zeroes on every BAR and ignoring writes.
> > > > >   \end{enumerate}
> > > > > 
> > > > > +\subsection{Virtio Structure PCI Extended Capabilities}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Extended Capabilities}
> > > > > +
> > > > > +The location of the virtio structures that depend on the PCI Express
> > > > > +capability are specified using a vendor-specific extended capabilities
> > > > > +on the extended capabilities list in PCI Express extended
> > > > > +configuration space of the device.
> > > > I can't parse this sentence. Can you rephrase it and/or split it into
> > > > multiple sentences?
> > > Sure, how about something like:
> > > 
> > > There could be virtio structures that depend on the PCI express
> > > capability. The location of those structures are specified using a
> > > vendor-specific extended capabilities on the extended capabilities
> > > list.
> > "virtio structures" -> "Virtio device configuration structures" (same as
> > in "4.1.4 Virtio Structure PCI Capabilities")
> 
> 
> Ok.
> 
> 
> > 
> > "PCI express capability" -> "PCI Express Extended Capability" or did you
> > mean that more generally (PCI Express features)?
> 
> 
> More general PCI Express feature.
> 
> 
> > 
> > Here is the edited sentence:
> > 
> >    Virtio device configuration structures that depend on PCI Express
> >    Extended Capabilities are specified using vendor-specific PCI Express
> >    Extended Capabilities located on the PCI Express Extended Capabilities
> >    list.
> 
> 
> That's fine.
> 
> 
> > > > > @@ -1488,6 +1611,10 @@ \subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virti
> > > > >   PCI capability list, detecting virtio configuration layout using Virtio
> > > > >   Structure PCI capabilities as detailed in \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities}
> > > > > 
> > > > > +Optionally, if the device is a PCIe device, the driver scans the PCI
> > > > > +Extended capability list, detecting virtio configuration layout using
> > > > > +Virtio Struct PCI Extended capabilities as detailed in \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Extended Capabilities}
> > > > If the same structure is present in both the PCI Capabilities and PCI
> > > > Extended Capabilities lists, which one has a higher priority?
> > > This is not allowed in this proposal where only the structure that
> > > depends on the PCI Express capability is allowed. E.g the PASID
> > > configuration structure depends on the PASID extended capability.
> > > 
> > > This is much more simpler than allowing existing virtio structures to
> > > be presented on both PCI and PCIe capability list.
> > Ah, I think I misunderstood the point of this patch. I thought you were
> > defining it because devices were running out of PCI Configuration Space
> > and PCI Extended Capabilities allow the device to use more space (PCI
> > Express Extended Configuration Space).
> > 
> > But now I think you're adding this to the spec in order to define VIRTIO
> > features that depend on PCI Extended Capabilities like PASID support?
> 
> 
> Right.
> 
> 
> > Is
> > it really necessary or just cleaner to use PCI Extended Capabilities
> > instead of VIRTIO's existing vendor-specific PCI Capabilities?
> 
> 
> Not a must, technically we can add them into the common configuration
> structure. But having a dedicated capability seems cleaner: it looks odd to
> configure PCIe features via the legacy capability anyhow. And this is more
> flexible to allow the hypervisor to map or hide the BAR independently.

Thanks for explaining. The purpose of the patch is clearer now.

Stefan

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