Re: [PATCH] x86/vtd: fix IOMMU share PT destruction path

["Jan Beulich" <JBeulich@suse.com>]

>>> On 09.10.18 at 14:57, <roger.pau@citrix.com> wrote:
> On Tue, Oct 09, 2018 at 12:22:34PM +0100, Paul Durrant wrote:
>> > -----Original Message-----
>> > From: Wei Liu [mailto:wei.liu2@citrix.com]
>> > Sent: 09 October 2018 11:42
>> > To: xen-devel@lists.xenproject.org 
>> > Cc: Jan Beulich <JBeulich@suse.com>; Roger Pau Monne
>> > <roger.pau@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>; Wei Liu
>> > <wei.liu2@citrix.com>; Kevin Tian <kevin.tian@intel.com>
>> > Subject: [PATCH] x86/vtd: fix IOMMU share PT destruction path
>> > 
>> > Commit 2916951c1 ("mm / iommu: include need_iommu() test in
>> > iommu_use_hap_pt()") included need_iommu() in iommu_use_hap_pt and
>> > 91d4eca7add (" mm / iommu: split need_iommu() into has_iommu_pt() and
>> > need_iommu_pt_sync()") made things finer grain by spliting need_iommu
>> > into three states.
>> > 
>> > The destruction path can't use iommu_use_hap_pt because at the point
>> > platform op is called, IOMMU is already switched to disabled state and
>> > the shared PT test would always be false.
>> > 
>> 
>> This is VT-d specific and the issue could affect AMD too. Can't you just 
> switch round the call to teardown and setting IOMMU_status_disabled in 
> xen/drivers/passthrough/iommu.c:iommu_teardown()?
> 
> AMD doesn't support shared page tables so far, but in order to avoid
> having to debug this again if AMD ever gains support for shared page
> tables I think switching the order in iommu_teardown might be better.

I think we've pretty much ruled out AMD ever re-gaining such
sharing functionality. In fact I thought we had settled that the
dead code relating to this functionality could be ripped out, if
anyone could carve out the time needed for doing so.

Jan



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