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Subject: Re: [PATCH] xen/pci: try to reserve MCFG areas earlier
To:     Jan Beulich <jbeulich@suse.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
CC:     <xen-devel@lists.xenproject.org>, <linux-kernel@vger.kernel.org>,
        <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>
References: <1567556431-9809-1-git-send-email-igor.druzhinin@citrix.com>
 <52fe7f67-ffd0-2d22-90fb-f3462ea059cd@suse.com>
From:   Igor Druzhinin <igor.druzhinin@citrix.com>
Message-ID: <d5dd94c2-070e-b3ff-57cf-92893b3cca7b@citrix.com>
Date:   Wed, 4 Sep 2019 12:36:02 +0100
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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?

Igor