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From: Bjorn Helgaas <bhelgaas@google.com>
Date: Thu, 4 Apr 2013 12:41:27 -0600
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Subject: Re: [PATCH v2] irq: add quirk for broken interrupt remapping on 55XX chipsets
To: Neil Horman <nhorman@tuxdriver.com>, Don Dutile <ddutile@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>, Prarit Bhargava <prarit@redhat.com>,
        Don Zickus <dzickus@redhat.com>, Jiang Liu <jiang.liu@huawei.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:INTEL IOMMU (VT-d)" <iommu@lists.linux-foundation.org>,
        Yinghai Lu <yinghai@kernel.org>
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On Thu, Apr 4, 2013 at 11:51 AM, Neil Horman <nhorman@tuxdriver.com> wrote:

> Oh, you want the bug report that I'm fixing this against?  Sure, I can do that.
> I thought you wanted me to include a url in the WARN_TAINT, with which user
> could report occurances of this bug.  Yeah, the bug that this is reported in is:
> https://bugzilla.redhat.com/show_bug.cgi?id=887006
>
> Its standing in for about a dozen or so variants of this issue we've seen

Exactly -- I'm just hoping for something in the changelog.  BTW, this
particular bugzilla is not public.

> Regardless, theres also the security issue to consider here - namely that
> disabling irq remapping opens up users of virt to a possible security bug
> (potential irq injection).  Some users may wish to live with the remapping
> error, given that error typically leads to devices that need to be
> restarted/reset to start working again, rather than live with the security hole.
> I rather like the warning, that gives users a choice, but I'll spin up a version
> that just disables it if you would rather.

I don't believe users will want to make a choice like that or even be
sophisticated enough to do it, at least not based on something in
dmesg.  I'm pretty sure I'm not  :)

The only supportable thing I can imagine doing would be:

  - Disable interrupt remapping if this chipset defect is present, so
devices work reliably (they don't need whatever restart/reset you
referred to above).
  - Disable virt functionality when interrupt remapping is disabled to
avoid the security problem (I don't know the details of this.)
  - Add a command-line option to enable interrupt remapping (I think
"intremap=on" is currently parsed too early, but maybe this could be
reworked so the option could override the quirk disable).
  - Add release notes saying "boot with 'intremap=on' if you want the
virt functionality and can accept unreliable devices."

That way the default behavior is safe and reliable (though perhaps
lacking some functionality), and you have told the user a way to get
safe and unreliable operation if he's willing to accept that.  At
least, that's what I think I would want if I were in RH's shoes.

Bjorn
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