Re: [Xen-devel] PCI passthrough to QEMU traditional stubdom not working when option ROM present

[Eric Shelton <eshelton@xxxxxxxxx>]

On Mon, Oct 24, 2016 at 9:12 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 24.10.16 at 15:01, <eshelton@xxxxxxxxx> wrote:
>> On Mon, Oct 24, 2016 at 8:47 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 24.10.16 at 14:30, <eshelton@xxxxxxxxx> wrote:
>>>> As best as I can tell, the current code already writes out the proper
>>>> value for sizing ("pci_write_long(d, reg, ~0)" in the above code
>>>> snippet).
>>>
>>> No - for sizing a ROM BAR, only the address portion is supposed to
>>> be written with all 1s as per the PCI spec. In particular it should be
>>> quite clear that writing the enable bit with 1 is a bad thing when the
>>> address portion is also written with all 1s.
>>>
>>> I'd like to note though that as of commit d2bd05d88d ("xen-pciback:
>>> return proper values during BAR sizing") the handling there is more
>>> relaxed - previously writes of 0xFFFFFFFE were the only ones
>>> considered to be sizing ones.
>>
>> Rev 2.2 of the PCI spec, at 6.2.5.2, states that "configuration
>> software can determine how much address space the device requires by
>> writing a value of all 1' to the address portion of the register."
>> Does the "address portion" refer to the upper 21 bits (meaning we
>> should write 0xFFFFF800) or the upper 31 bits (meaning we should write
>> 0xFFFFFFFE)?
>
> Clearly the upper 21 bits.

It's perhaps not all that clear, given that earlier pciback code
expected 0xFFFFFFFE.

>>  4.4.14 pciback code appears to expect the latter, and
>> with the newer more relaxed pciback code it doesn't matter.  For
>> purposes of fixing this bug in Xen, which do you consider to be the
>> "correct" sizing write value for QEMU to be writing out when it sizes
>> the expansion ROM?
>
> 0xFFFFF800 | (<read-val> & 0x7FE)

Excuse me for being a little dense, but I don't understand what is
going on with the "| (<read-val> & 0x7FE)" portion above.  Should the
fixed code be:

pci_write_long(d, reg, ~PCI_ROM_ADDRESS_MASK);

Or something else?

Thanks,
Eric

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