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Subject: Re: [PATCH] xen/pci: try to reserve MCFG areas earlier
To:     Igor Druzhinin <igor.druzhinin@citrix.com>
Cc:     Andrew Cooper <andrew.cooper3@citrix.com>,
        xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
        Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org
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 <d5dd94c2-070e-b3ff-57cf-92893b3cca7b@citrix.com>
From:   Jan Beulich <jbeulich@suse.com>
Message-ID: <d2bcdab9-7bf5-9266-eadf-5c5ab4de7105@suse.com>
Date:   Wed, 4 Sep 2019 14:09:37 +0200
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On 04.09.2019 13:36, Igor Druzhinin wrote:
> On 04/09/2019 10:08, Jan Beulich wrote:
>> On 04.09.2019 02:20, Igor Druzhinin wrote:
>>> If MCFG area is not reserved in E820, Xen by default will defer its usage
>>> until Dom0 registers it explicitly after ACPI parser recognizes it as
>>> a reserved resource in DSDT. Having it reserved in E820 is not
>>> mandatory according to "PCI Firmware Specification, rev 3.2" (par. 4.1.2)
>>> and firmware is free to keep a hole E820 in that place. Xen doesn't know
>>> what exactly is inside this hole since it lacks full ACPI view of the
>>> platform therefore it's potentially harmful to access MCFG region
>>> without additional checks as some machines are known to provide
>>> inconsistent information on the size of the region.
>>
>> Irrespective of this being a good change, I've had another thought
>> while reading this paragraph, for a hypervisor side control: Linux
>> has a "memopt=" command line option allowing fine grained control
>> over the E820 map. We could have something similar to allow
>> inserting an E820_RESERVED region into a hole (it would be the
>> responsibility of the admin to guarantee no other conflicts, i.e.
>> it should generally be used only if e.g. the MCFG is indeed known
>> to live at the specified place, and being properly represented in
>> the ACPI tables). Thoughts?
> 
> What other use cases can you think of in case we'd have this option?
> From the top of my head, it might be providing a memmap for a second Xen
> after doing kexec from Xen to Xen.
> 
> What benefits do you think it might have over just accepting a hole
> using "mcfg=relaxed" option from admin perspective?

It wouldn't be MCFG-specific, i.e. it could also be used to e.g.
convert holes to E820_RESERVED to silence VT-d's respective RMRR
warning. Plus by inserting the entry into our own E820 we'd also
propagate it to users of XENMEM_machine_memory_map.

Jan