linux-kernel - Re: [PATCH] iommu/vt-d: Check VT-d RMRR region in BIOS is reported as reserved

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Message-ID: <64a5843d-850d-e58c-4fc2-0a0eeeb656dc@nec.com>
Date:   Tue, 18 Feb 2020 08:28:34 +0000
From:   NOMURA JUNICHI(野村　淳一) 
        <junichi.nomura@....com>
To:     Yian Chen <yian.chen@...el.com>
CC:     "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "sohil.mehta@...el.com" <sohil.mehta@...el.com>,
        "jroedel@...e.de" <jroedel@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH] iommu/vt-d: Check VT-d RMRR region in BIOS is reported as
 reserved

Hi,

On 10/16/19 1:49 AM, Yian Chen wrote:
> VT-d RMRR (Reserved Memory Region Reporting) regions are reserved
> for device use only and should not be part of allocable memory pool of OS.
> 
> BIOS e820_table reports complete memory map to OS, including OS usable
> memory ranges and BIOS reserved memory ranges etc.
> 
> x86 BIOS may not be trusted to include RMRR regions as reserved type
> of memory in its e820 memory map, hence validate every RMRR entry
> with the e820 memory map to make sure the RMRR regions will not be
> used by OS for any other purposes.

I found "bad RMRR" warnings starting to appear on some x86 servers
since v5.5-rc1 and it gets even louder since v5.6-rc1.  The "bad"
RMRR is for the area resides in ACPI NVS memory region.  For example,

# dmesg|grep RMRR
DMAR: RMRR base: 0x000000a2290000 end: 0x000000a2292fff
DMAR: [Firmware Bug]: No firmware reserved region can cover this RMRR [0x00000000a2290000-0x00000000a2292fff], contact BIOS vendor for fixes
Your BIOS is broken; bad RMRR [0x00000000a2290000-0x00000000a2292fff]

# dmesg|grep NVS
BIOS-e820: [mem 0x00000000a067a000-0x00000000a2a79fff] ACPI NVS
reserve setup_data: [mem 0x00000000a067a000-0x00000000a2a79fff] ACPI NVS
PM: Registering ACPI NVS region [mem 0xa067a000-0xa2a79fff] (37748736 bytes)

The warnings come from arch_rmrr_sanity_check() since it checks whether
the region is E820_TYPE_RESERVED.  However, if the purpose of the check
is to detect RMRR has regions that may be used by OS as free memory,
isn't E820_TYPE_NVS safe, too?

-- 
Jun'ichi Nomura, NEC Corporation / NEC Solution Innovators, Ltd.