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

[Weidong Han <weidong.han@xxxxxxxxx>]

Noboru Iwamatsu wrote:
> Hi Weidong,
>
>   
> > I implemented a patch for it. Noboru, pls have a try on your machine.
> > If you use default iommu=1, VT-d will be disabled with warning messages.
> > If you use iommu=workaround_bios_bug, it should enable VT-d and works
> > for you.
> > If you use iommu=force, it panics.
> >     
> On my machine, each options have worked as described.
>   
Thanks Noboru.
> I tried:
> xen-unstable c/s 20844 + drhd-ignore.patch + workaround-bios.patch
>   
drhd-ignore.patch was already checked in as c/s 20846. Keir, pls check 
in the workaround-bios.patch. Thanks.
Regards,
Weidong
> Thanks,
> Noboru.
>
>   
> > patch title: VT-d: add "iommu=workaround_bios_bug" option
> > patch description:
> > Add this option to workaround BIOS bugs. Currently it ignores DRHD if
> > "all" devices under its scope are not pci discoverable. This workarounds
> > a BIOS bug in some platforms to make VT-d work. But note that this
> > option doesn't guarantee security, because it might ignore DRHD.
> > So there are 3 options which handle BIOS bugs differently:
> > iommu=1 (default): If detect non-existent device under a DRHD's scope,
> > or find incorrect RMRR setting (base_address > end_address), disable
> > VT-d completely in Xen with warning messages. This guarantees security
> > when VT-d enabled, or just disable VT-d to let Xen work without VT-d.
> > iommu=force: it enforces to enable VT-d in Xen. If VT-d cannot be
> > enabled, it will crashes Xen. This is mainly for users who must need VT-d.
> > iommu=workaround_bogus_bios: it workarounds some BIOS bugs to make VT-d
> > still work. This might be insecure because there might be a device not
> > protected by any DRHD if the device is re-enabled by malicious s/w. This
> > is for users who want to use VT-d regardless of security.
> >
> > Signed-off-by: Weidong Han <weidong.han@xxxxxxxxx>
> >
> > Regards,
> > Weidong
> >
> > Noboru Iwamatsu wrote:
> >     
> > > Weidong, Keir,
> > >
> > > I agree your suggestions.
> > >
> > > Noboru.
> > >
> > >       
> > > > Keir Fraser wrote:
> > > >         
> > > > > On 25/01/2010 10:45, "Sander Eikelenboom" <linux@xxxxxxxxxxxxxx> wrote:
> > > > >
> > > > >           
> > > > > > a) Could be discussed if panic should be default instead of disabling
> > > > > > iommu or
> > > > > > not, although there seem to be a lot of broken bioses, so that would
> > > > > > lead to a
> > > > > > lot of machines not booting.
> > > > > >             
> > > > > Absolutely not acceptable. Warn and completely disable IOMMU is the
> > > > > correct
> > > > > default causing least pain to the most end users.
> > > > >
> > > > > -- Keir
> > > > >
> > > > >           
> > > > Agree. It should not crash Xen by default due to BIOS issues.
> > > > warn-and-disable is better. It won't impact common Xen users, and if a
> > > > user really wants to use VT-d, he can try iommu=workaround_bogus_bios,
> > > > or directly report to OEM vendor to get it fixed in BIOS. As VT-d is
> > > > used more and more widely, I think the BIOS issues will be found and
> > > > fixed more quickly than before, thus the situation should be better.
> > > >
> > > > Regards,
> > > > Weidong
> > > >
> > > >
> > > >
> > > >         
> > >       
>
>   

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