Re: [Xen-devel] [PATCH v3.1 07/15] xen/x86: do the PCI scan unconditionally

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.11.16 at 11:58, <roger.pau@xxxxxxxxxx> wrote:
> On Mon, Oct 31, 2016 at 10:47:15AM -0600, Jan Beulich wrote:
>> >>> On 29.10.16 at 10:59, <roger.pau@xxxxxxxxxx> wrote:
>> > --- a/xen/arch/x86/setup.c
>> > +++ b/xen/arch/x86/setup.c
>> > @@ -1491,6 +1491,8 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>> >  
>> >      early_msi_init();
>> >  
>> > +    scan_pci_devices();
>> > +
>> >      iommu_setup();    /* setup iommu if available */
>> >  
>> >      smp_prepare_cpus(max_cpus);
>> > --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
>> > +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
>> > @@ -219,7 +219,8 @@ int __init amd_iov_detect(void)
>> >  
>> >      if ( !amd_iommu_perdev_intremap )
>> >          printk(XENLOG_WARNING "AMD-Vi: Using global interrupt remap table 
>> > is not recommended (see XSA-36)!\n");
>> > -    return scan_pci_devices();
>> > +
>> > +    return 0;
>> >  }
>> 
>> I'm relatively certain that I did point out on a prior version that the
>> error handling here gets lost. At the very least the commit message
>> should provide a reason for doing so; even better would be if there
>> was no behavioral change (other than the point in time where this
>> happens slightly changing).
> 
> Behaviour here is different on Intel or AMD hardware, on Intel failure to 
> scan the PCI bus will not be fatal, and the IOMMU will be enabled anyway. On 
> AMD OTOH failure to scan the PCI bus will cause the IOMMU to be disabled. 
> I expect we should be able to behave equally for both Intel and AMD, so 
> which one should be used?

I'm afraid I have to defer to the vendor IOMMU maintainers for
that one, as I don't know the reason for the difference in behavior.
An aspect that may play into here is that for AMD the IOMMU is
represented by a PCI device, while for Intel it's just a part of one
of the core chipset devices.

Jan


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