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From: Igor Druzhinin <igor.druzhinin@citrix.com>
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>
Subject: Re: [PATCH] xen/pci: try to reserve MCFG areas earlier
Date: Wed, 4 Sep 2019 12:36:02 +0100	[thread overview]
Message-ID: <d5dd94c2-070e-b3ff-57cf-92893b3cca7b@citrix.com> (raw)
In-Reply-To: <52fe7f67-ffd0-2d22-90fb-f3462ea059cd@suse.com>

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

WARNING: multiple messages have this Message-ID (diff)
From: Igor Druzhinin <igor.druzhinin@citrix.com>
To: Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Juergen Gross <jgross@suse.com>,
	xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
	linux-kernel@vger.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier
Date: Wed, 4 Sep 2019 12:36:02 +0100	[thread overview]
Message-ID: <d5dd94c2-070e-b3ff-57cf-92893b3cca7b@citrix.com> (raw)
In-Reply-To: <52fe7f67-ffd0-2d22-90fb-f3462ea059cd@suse.com>

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

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Xen-devel@lists.xenproject.org
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  reply	other threads:[~2019-09-04 11:43 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04  0:20 [PATCH] xen/pci: try to reserve MCFG areas earlier Igor Druzhinin
2019-09-04  0:20 ` [Xen-devel] " Igor Druzhinin
2019-09-04  9:08 ` Jan Beulich
2019-09-04  9:08   ` [Xen-devel] " Jan Beulich
2019-09-04 11:36   ` Igor Druzhinin [this message]
2019-09-04 11:36     ` Igor Druzhinin
2019-09-04 12:09     ` Jan Beulich
2019-09-04 12:09       ` [Xen-devel] " Jan Beulich
2019-09-06 22:30 ` Boris Ostrovsky
2019-09-06 22:30   ` [Xen-devel] " Boris Ostrovsky
2019-09-06 23:00   ` Igor Druzhinin
2019-09-06 23:00     ` [Xen-devel] " Igor Druzhinin
2019-09-08 18:28     ` Boris Ostrovsky
2019-09-08 18:28       ` [Xen-devel] " Boris Ostrovsky
2019-09-08 21:11       ` Igor Druzhinin
2019-09-08 21:11         ` [Xen-devel] " Igor Druzhinin
2019-09-08 23:30         ` Boris Ostrovsky
2019-09-08 23:30           ` [Xen-devel] " Boris Ostrovsky
2019-09-08 23:37           ` Igor Druzhinin
2019-09-08 23:37             ` [Xen-devel] " Igor Druzhinin
2019-09-09 19:19             ` Boris Ostrovsky
2019-09-09 19:19               ` [Xen-devel] " Boris Ostrovsky
2019-09-09 21:48               ` Igor Druzhinin
2019-09-09 21:48                 ` [Xen-devel] " Igor Druzhinin
2019-09-10  1:47                 ` Boris Ostrovsky
2019-09-10  1:47                   ` Boris Ostrovsky
2019-09-10  9:46                   ` Igor Druzhinin
2019-09-10  9:46                     ` Igor Druzhinin
2019-09-10  9:55                     ` Jan Beulich
2019-09-10  9:55                       ` Jan Beulich
2019-09-10 10:08                       ` Igor Druzhinin
2019-09-10 10:08                         ` Igor Druzhinin
2019-09-10 17:48                     ` Boris Ostrovsky
2019-09-10 17:48                       ` Boris Ostrovsky
2019-09-10 20:36                       ` Igor Druzhinin
2019-09-10 20:36                         ` Igor Druzhinin
2019-09-10 21:19                         ` Boris Ostrovsky
2019-09-10 21:19                           ` Boris Ostrovsky
2019-09-11  1:15                           ` Igor Druzhinin
2019-09-11  1:15                             ` Igor Druzhinin
2019-09-11  9:13                             ` Jan Beulich
2019-09-11  9:13                               ` Jan Beulich
2019-09-12 17:33                             ` Boris Ostrovsky
2019-09-12 17:33                               ` Boris Ostrovsky

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