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Re: [Xen-devel] [PATCH] VT-d: improve RMRR validity checking

Noboru Iwamatsu wrote:

I read the patch and the following thread.

I understood what you mean, but I think it's better to
limit the scope of "force_iommu".
And I believe RMRR should be checked as same as DRHD.

What I thought about DRHD is:
If all devices under the scope of the DRHD are non-existent,
this DRHD is invalid but safely ignorable, so ignore it.
No, we cannot ignore it if iommu=force. The invisible device may be disabled, not really non-existent. it is possibly that it is re-enabled by malfunctional s/w. So when iommu=force, we should not ignore any DRHD. We ignores it just to workaround the BIOS issue you encountered.
If some devices under the scope of the DRHD are non-existent,
this DRHD is invalid, so disable VT-d unless "iommu=force"
option is specified.
When "iommu=force" option is specified, even the invalid DRHD
will be registered, because DRHD that has some existent devices
must not be ignored due to security reasons.

About the RMRR:
If all devices under the scope of the RMRR are non-existent,
this RMMR is invalid but ignorable, so ignore it.
If some devices under the scope of the RMRR are non-existent,
this RMRR is invalid, so disable VT-d unless "iommu=force"
RMRR is much different from DRHD, it's just reversed memories for specific devices (now only Intel IGD and USB contollers need RMRR), it's no security issue like described above. if "all" devices under the scope of the RMRR are non-existent, we can ignore the RMRR because no devices will use it. if some" devices under the scope of the RMRR are non-existent, we cannot ignore the RMRR, because there are still some devices want to use it. I think we needn't to disable VT-d because it won't cause any issues. Of course, we also can disable VT-d for this case strictly.
option is specified. When "iommu=force" option is specified,
the invalid RMRR is ignored (it's safe).

I attach the patch.

What do you think?


I think it need not to change current code. BTW, your patch is not based on latest Xen.



I implemented a patch and attached.

patch description:
In order to make Xen more defensive to VT-d related BIOS issue, this
patch ignores a DRHD if all devices under its scope are not pci
discoverable, and regards a DRHD as invalid and then disable whole VT-d
if some devices under its scope are not pci discoverable. But if
iommu=force is set, it will enable all DRHDs reported by BIOS, to avoid
any security vulnerability with malicious s/s re-enabling "supposed
disabled" devices. Pls note that we don't know the devices under the
"Include_all" DRHD are existent or not, because the scope of
"Include_all" DRHD won't enumerate common pci device, it only enumerates
I/OxAPIC and HPET devices.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>
Signed-off-by: Weidong Han <weidong.han@xxxxxxxxx>

Noboru, pls test the patch on your machine?

Joe, could you review the patch? and pls ACK it if it's fine for you.


Noboru Iwamatsu wrote:

I understood.

Noboru Iwamatsu wrote:
Hi Weidong,

I'm not sure why the security problem is caused by ignoring
the DRHD that has only non-existent devices.

Could you explain details or where to read the spec?
It's requested from security experts. The device that is not pci
discoverable may be re-enabled by malicious software. If its DRHD is not
enabled, the re-enabled device is not protected by VT-d. It will cause
security issue.

As you saying, security is the top-priority.
However, when iommu=force is specified, we should enable vt-d
if there are some potential issues.
Because users want to "force" anyway.
iommu=force was introduced to enable VT-d anyway for security purpose. I
plan to still enable those DRHDs that includes non-existed device when
iommu=force, otherwise ignore them.


Keir Fraser wrote:
If we want to keep iommu=1 as default, then it is unacceptable to
fail to
boot on a fairly wide range of modern systems. We have to
partially or completely, unless iommu=force is specified. Or we
need to
revert to iommu=0 as the default.

What do you think, Weidong?
Yes. I agree to warn-and-disable for these BIOS issues, and consider
security more when iommu=force. Therefore I will implement a patch
on Nororu's patch.


-- Keir

On 21/01/2010 14:17, "Sander Eikelenboom" <linux@xxxxxxxxxxxxxx>

Hello Weidong,

The problem is most vendor's just don't fix it and ignore the
Most often hiding them selves behind: come back when it's a problem
Microsoft Windows, that the only single thing we support (and no
software, so no vmware, no xen, no linux, perhaps even no
Well I don't know if the virtual pc in windows 7 supports an iommu
now, but it
didn't in the past as far as i know, so any complain bounces off,
there it
all seems to end for them.

Besides that i don't know if they do know what the problems with
implementation in BIOS is when someone reports it.
I think some behind the scenes pressure from Intel to vendors might
help to
solve some of them.
(my Q35 chipset, "Intel V-PRO" marketed motherboard (so much for
that) also
suffers RMRR problem when another graphics card is inserted which
switches off
the IGD).

Although i think in my case your patch will work around that for me.
Perhaps a
third option is needed, which does all the workarounds possible and
about potential security problem when requested ?


Thursday, January 21, 2010, 1:46:39 PM, you wrote:

Noboru Iwamatsu wrote:
Hi Weidong,

I re-send the DRHD-fix patch.

If DRHD does not have existent devices, ignore it.
If DRHD has both existent and non-existent devices, consider it
and not register.
Although you patch workarounds your buggy BIOS, but we still
need to
enable it for security purpose as I mentioned in previous mail. We
needn't workaround / fix all BIOS issues in software. I think
is more important for this specific BIOS issue. Did you report the
issue to your OEM vendor? maybe it's better to get it fixed in
According to this patch and yours, my machine successfully booted
with vt-d enabled.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@xxxxxxxxxxxxxx>

Keir Fraser wrote:
On 21/01/2010 10:19, "Weidong Han" <weidong.han@xxxxxxxxx>

Sorry this is typo.
I mean:
So, I think RMRR that has no-existent device is "invalid"
and whole RMRR should be ignored.
looks reasonable.

Keir, I Acks Noboru's rmrr patch. Or do you want us to merge
them to one
Merge them up, re-send with both sign-off and acked-by all in


Sorry, I disagree with Noboru after thinking it again. If the
both no-existent device and also has existent devices in its
scope, we
should not ignore it because the existent devices under its scope
be impacted without the RMRR. so I suggest to print a warning
instead of
ignore it. Attached a patch for it.

Signed-off-by: Weidong Han <weidong.han@xxxxxxxxx>

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