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

On Tue, Oct 09, 2018 at 12:22:34PM +0100, Paul Durrant wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: 09 October 2018 11:42
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Jan Beulich <JBeulich@xxxxxxxx>; Roger Pau Monne
> > <roger.pau@xxxxxxxxxx>; Paul Durrant <Paul.Durrant@xxxxxxxxxx>; Wei Liu
> > <wei.liu2@xxxxxxxxxx>; Kevin Tian <kevin.tian@xxxxxxxxx>
> > 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.

Thanks, Roger.

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