Re: [Xen-devel] xsa46-4.2.patch breaks PCI passthrough?

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.05.13 at 13:28, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/05/13 12:09, Andrew Cooper wrote:
>> On 01/05/13 06:29, Steven Haigh wrote:
>>> Hi all,
>>>
>>> I've had a report lodged against my packages that the patch provided for 
>>> XSA46 against Xen 4.2.1 causes PCI passthru to break.
>>>
>>> It seems that 4.2.1 *without* the XSA46 patch works perfectly. 4.2.2 
>>> does not work.
>>>
>>> I added this patch in xen-4.2.1-6 of my RPMs (http://xen.crc.id.au) and 
>>> the reporter has built the same SRPM with xsa46 patch removed and PCI 
>>> passthrough works as intended.
>>>
>>> Reapplying the XSA46 patch causes it to break again.
>>>
>>> The bug report and logs can be found here:
>>>     http://xen.crc.id.au/bugs/view.php?id=5 
>>>
>>> Has anyone come across this?
>>>
>> XSA-46 was to do with PCI passthrough of PV domains, and in particular
>> changing some of the rules regarding interrupts.

This was misguiding me - I somehow concluded that the problems
here are being observed with PV domains, but considering the
second report we got as well as looking through the log files I'm
now rather guessing that the problem is (only) with HVM domains.
That in turn would match up with the code in pciif.py:

        if not self.vm.info.is_hvm() and dev.irq:
            rc = xc.physdev_map_pirq(domid = fe_domid,
                                   index = dev.irq,
                                   pirq  = dev.irq)
            if rc < 0:
                raise VmError(('pci: failed to map irq on device '+
                            '%s - errno=%d')%(dev.name,rc))
        if dev.irq>0:
            log.debug('pci: enabling irq %d'%dev.irq)
            rc = xc.domain_irq_permission(domid =  fe_domid, pirq = dev.irq,
                    allow_access = True)
            if rc<0:
                raise VmError(('pci: failed to configure irq on device '+
                            '%s - errno=%d')%(dev.name,rc))

i.e. the first portion of the setup is only being done for PV
guests. I have no idea why this is so (irqif.py doesn't special
case the guest kind, nor does libxl). Quite likely dropping that
check would be sufficient, but of course that should be
confirmed by someone knowing that code (and ideally also
knowing why this was being special cased in the first place) -
Ian, Ian?

(Oddly enough, the first check also does "dev.irq != 0) while
the second uses "dev.irq > 0" - I wouldn't expect negative
values to ever appear here, but it's another hint at there
being unnecessary inconsistencies here.)

Jan


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