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

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Message-ID: <74c9d2cc-a528-2cec-099e-0d803aeace6f@citrix.com>
Date:   Wed, 11 Sep 2019 02:15:13 +0100
From:   Igor Druzhinin <igor.druzhinin@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
CC:     <jgross@...e.com>
Subject: Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier

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.
> 

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