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

On Wed, Oct 10, 2012 at 08:36:12AM -0500, Anthony Liguori wrote:

Rusty Russell [off-list ref] writes:

quoted

Gerd Hoffmann [off-list ref] writes:

quoted

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.

quoted

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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