Re: [Qemu-devel] Using PCI config space to indicate config location

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

On Wed, Oct 10, 2012 at 08:36:12AM -0500, Anthony Liguori wrote:
> Rusty Russell <rusty@rustcorp.com.au> writes:
> 
> > Gerd Hoffmann <kraxel@redhat.com> writes:
> >> So how about this:
> >>
> >> (1) Add a vendor specific pci capability for new-style virtio.
> >>     Specifies the pci bar used for new-style virtio registers.
> >>     Guests can use it to figure whenever new-style virtio is
> >>     supported and to map the correct bar (which will probably
> >>     be bar 1 in most cases).
> >
> > This was closer to the original proposal[1], which I really liked (you
> > can layout bars however you want).  Anthony thought that vendor
> > capabilities were a PCI-e feature, but it seems they're blessed in PCI
> > 2.3.
> 
> 2.3 was standardized in 2002.  Are we confident that vendor extensions
> play nice with pre-2.3 OSes like Win2k, WinXP, etc?
> 
> I still think it's a bad idea to rely on something so "new" in something
> as fundamental as virtio-pci unless we have to.
> 
> Regards,
> 
> Anthony Liguori

Pre 2.3 it was the case that *all* space outside
the capability linked list, and any capability not
recognized by space, was considered vendor specific.
So there's no problem really.

> >
> > So let's return to that proposal, giving something like this:
> >
> > /* IDs for different capabilities.  Must all exist. */
> > /* FIXME: Do we win from separating ISR, NOTIFY and COMMON? */
> > /* Common configuration */
> > #define VIRTIO_PCI_CAP_COMMON_CFG	1
> > /* Notifications */
> > #define VIRTIO_PCI_CAP_NOTIFY_CFG	2
> > /* ISR access */
> > #define VIRTIO_PCI_CAP_ISR_CFG		3
> > /* Device specific confiuration */
> > #define VIRTIO_PCI_CAP_DEVICE_CFG	4
> >
> > /* This is the PCI capability header: */
> > 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: sizeof(struct virtio_pci_cap). */
> > 	u8 cfg_type;	/* One of the VIRTIO_PCI_CAP_*_CFG. */
> > 	u8 bar;		/* Where to find it. */
> > 	u8 unused;
> > 	__le16 offset;	/* Offset within bar. */
> > 	__le32 length;	/* Length. */
> > };
> >
> > This means qemu can point the isr_cfg into the legacy area if it wants.
> > In fact, it can put everything in BAR0 if it wants.
> >
> > Thoughts?
> > Rusty.
> > --
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