Re: [Xen-devel] [PATCH v7] VT-d: use correct BDF for VF to search VT-d unit

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Aug 23, 2017 at 01:20:13AM -0600, Jan Beulich wrote:
> >>> On 23.08.17 at 09:16, <roger.pau@xxxxxxxxxx> wrote:
> > On Wed, Aug 23, 2017 at 09:05:14AM +0800, Chao Gao wrote:
> >> On Tue, Aug 22, 2017 at 06:43:49AM -0600, Jan Beulich wrote:
> >> >>>> On 21.08.17 at 23:52, <chao.gao@xxxxxxxxx> wrote:
> >> >> --- a/xen/include/xen/pci.h
> >> >> +++ b/xen/include/xen/pci.h
> >> >> @@ -39,6 +39,10 @@
> >> >>  #define PCI_SBDF3(s,b,df) ((((s) & 0xffff) << 16) | PCI_BDF2(b, df))
> >> >>  
> >> >>  struct pci_dev_info {
> >> >> +    /*
> >> >> +     * When 'is_virtfn' is set, 'is_extfn' is re-used to indicate 
> >> >> whether
> >> >> +     * the PF of this VF is an extended function.
> >> >> +     */
> >> >
> >> >I'd be inclined to extend the comment by appending ", as a VF itself
> >> >can never be an extended function." Is that correct? If so, would
> >> 
> >> Hi, Jan and Roger.
> >> 
> >> Strictly speaking, the VF can be an extended function. The definition is
> >> within ARI device (in this kind of device, device field is treated as an
> >> extension of function number) and function number is greater than 7. But
> >> this field isn't used as we don't care about whether a VF is or not an
> >> extended function (at least at present).
> >> 
> >> Eric reviewed this patch and told me we may match
> >> 'if ( pdev->info.is_extfn )' in acpi_find_matched_drhd_unit.
> >> So we may introduce a new field like what I do in v6 or check
> >> 'pdev->info.is_virtfn' first in acpi_find_matched_drhd_unit (maybe other
> >> places we check pdev->info.is_extfn).
> >> 
> >> Which one do you prefer?
> > 
> > Looking at this again I'm not sure why you need any modifications to
> > acpi_find_matched_drhd_unit. If the virtual function is an extended
> > function pdev->bus should be equal to pdev->info.physfn.bus, in which
> > case the already existing is_extfn check will already DTRT?
> > 
> > Ie: an extended VF should always have the same bus as the PF it
> > belongs to, unless I'm missing something.
> 
> Why would that be?

It is my understanding (which might be wrong), that an extended
function simply uses 8 bits for the function number, which on a
traditional device would be used for both the slot and the function
number.

So extended functions have no slot, but the bus number is the same for
all of them, or else they would belong to different devices due to the
difference in the bus numbers.

Maybe what I'm missing is whether it is possible to have a device with
virtual functions that expand across several buses?

Roger.

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