Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier

From: Boris Ostrovsky
Date: Thu Sep 12 2019 - 13:34:42 EST

On 9/10/19 9:15 PM, Igor Druzhinin wrote:
> On 10/09/2019 22:19, Boris Ostrovsky wrote:
>> On 9/10/19 4:36 PM, Igor Druzhinin wrote:
>>> On 10/09/2019 18:48, Boris Ostrovsky wrote:
>>>> On 9/10/19 5:46 AM, Igor Druzhinin wrote:
>>>>> On 10/09/2019 02:47, Boris Ostrovsky wrote:
>>>>>> On 9/9/19 5:48 PM, Igor Druzhinin wrote:
>>>>>>> On 09/09/2019 20:19, Boris Ostrovsky wrote:
>>>>>>>> The other question I have is why you think it's worth keeping
>>>>>>>> xen_mcfg_late() as a late initcall. How could MCFG info be updated
>>>>>>>> between acpi_init() and late_initcalls being run? I'd think it can only
>>>>>>>> happen when a new device is hotplugged.
>>>>>>> It was a precaution against setup_mcfg_map() calls that might add new
>>>>>>> areas that are not in MCFG table but for some reason have _CBA method.
>>>>>>> It's obviously a "firmware is broken" scenario so I don't have strong
>>>>>>> feelings to keep it here. Will prefer to remove in v2 if you want.
>>>>>> Isn't setup_mcfg_map() called before the first xen_add_device() which is where you are calling xen_mcfg_late()?
>>>>> setup_mcfg_map() calls are done in order of root bus discovery which
>>>>> happens *after* the previous root bus has been enumerated. So the order
>>>>> is: call setup_mcfg_map() for root bus 0, find that
>>>>> pci_mmcfg_late_init() has finished MCFG area registration, perform PCI
>>>>> enumeration of bus 0, call xen_add_device() for every device there, call
>>>>> setup_mcfg_map() for root bus X, etc.
>>>> Ah, yes. Multiple busses.
>>>> If that's the case then why don't we need to call xen_mcfg_late() for
>>>> the first device on each bus?
>>> Ideally, yes - we'd like to call it for every bus discovered. But boot
>>> time buses are already in MCFG (otherwise system boot might not simply
>>> work as Jan pointed out) so it's not strictly required. The only case is
>>> a potential PCI bus hot-plug but I'm not sure it actually works in
>>> practice and we certainly didn't support it before. It might be solved
>>> theoretically by subscribing to acpi_bus_type that is available after
>>> acpi_init().
>> OK. Then *I think* we can drop late_initcall() but I would really like
>> to hear when others think.

Since noone commented then can you send a v2 with second patch removing
the late call?

Also, in the first patch please limit the scope of pci_mcfg_reserved to
just xen_add_device().


> Another thing that I implied by "not supporting" but want to explicitly
> call out is that currently Xen will refuse reserving any MCFG area
> unless it actually existed in MCFG table at boot. I don't clearly
> understand reasoning behind it but it might be worth relaxing at least
> size matching restriction on Xen side now with this change.
> Igor
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