Re: [Xen-devel] [PATCH] VTD/Intremap: Disable Intremap on Chipset 5500/5520/X58 due to errata

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.01.13 at 10:15, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Thu, 2013-01-17 at 09:09 +0000, Jan Beulich wrote:
>> >>> On 17.01.13 at 10:01, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
>> > On Thu, 2013-01-17 at 08:49 +0000, Jan Beulich wrote:
>> >> So the question really is whether, rather than disabling interrupt
>> >> remapping, disabling the IOMMU as a whole wouldn't be the
>> >> better (read: more secure) workaround. 
>> > 
>> > That, plus providing a command line "I accept the risks" option, is the
>> > approach we've taken in the past I think.
>> 
>> An option for this we already have - "iommu=force".
> 
> I thought that but isn't it sort of the other way found? iommu=force
> will fail to boot if the iommu cannot be enabled, including intremap
> support.

Of course using "iommu=force" when enabling the IOMMU fails
is a bad idea - that will indeed panic. But we're talking about the
situation where enabling the IOMMU so far worked fine, and the
option is only to be used as an override to us disabling it as
workaround.

> But perhaps iommu=no-intremap does what we want already?

Not really - we want to do the disabling automatically (turning
off both iommu and interrupt remapping), so that to override
this one would need "iommu=force" (enabling the IOMMU, but
not interrupt remapping) or "iommu=force,intremap" (enabling
both - that case needs some code adjustment to work as
described).

Jan


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