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

[Chao Gao <chao.gao@xxxxxxxxx>]

On Wed, Aug 23, 2017 at 09:01:07AM +0100, Roger Pau Monné wrote:
>On Wed, Aug 23, 2017 at 02:46:08PM +0800, Chao Gao wrote:
>> On Wed, Aug 23, 2017 at 08:31:51AM +0100, Roger Pau Monné wrote:
>> >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?
>> 
>> It is not true. Please refer to the 2.1.2 VF Discovery of SR-IOV spec.
>> The numbers of VF can be larger than 256 and so it is definite that
>> sometimes VF's bus number would be different from the PF's.
>
>So that's what I was missing, thanks.
>
>Then I would modify acpi_find_matched_drhd_unit so it's:
>
>    if ( pdev->info.is_extfn )
>    {
>        bus = pdev->info.is_virtfn ? pdev->info.physfn.bus : pdev->bus;
>        devfn = 0;
>    }
>
>AFAICT that should work?

Fine to me.

Jan, What your opinion on this piece of code?

Thanks
Chao

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