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

[Anthony Liguori <aliguori@us.ibm.com>]

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

>
> 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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