Re: [PATCH 5/7] public / x86: introduce __HYPERCALL_iommu_op

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Tian, Kevin [mailto:kevin.tian@intel.com]
> Sent: 13 February 2018 06:43
> To: Paul Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org
> Cc: Stefano Stabellini <sstabellini@kernel.org>; Wei Liu
> <wei.liu2@citrix.com>; George Dunlap <George.Dunlap@citrix.com>;
> Andrew Cooper <Andrew.Cooper3@citrix.com>; Ian Jackson
> <Ian.Jackson@citrix.com>; Tim (Xen.org) <tim@xen.org>; Jan Beulich
> <jbeulich@suse.com>; Daniel De Graaf <dgdegra@tycho.nsa.gov>
> Subject: RE: [Xen-devel] [PATCH 5/7] public / x86: introduce
> __HYPERCALL_iommu_op
> 
> > From: Paul Durrant
> > Sent: Monday, February 12, 2018 6:47 PM
> >
> > This patch introduces the boilerplate for a new hypercall to allow a
> > domain to control IOMMU mappings for its own pages.
> > Whilst there is duplication of code between the native and compat entry
> > points which appears ripe for some form of combination, I think it is
> > better to maintain the separation as-is because the compat entry point
> > will necessarily gain complexity in subsequent patches.
> >
> > NOTE: This hypercall is only implemented for x86 and is currently
> >       restricted by XSM to dom0 since it could be used to cause IOMMU
> >       faults which may bring down a host.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> [...]
> > +
> > +
> > +static bool can_control_iommu(void)
> > +{
> > +    struct domain *currd = current->domain;
> > +
> > +    /*
> > +     * IOMMU mappings cannot be manipulated if:
> > +     * - the IOMMU is not enabled or,
> > +     * - the IOMMU is passed through or,
> > +     * - shared EPT configured or,
> > +     * - Xen is maintaining an identity map.
> 
> "for dom0"
> 
> > +     */
> > +    if ( !iommu_enabled || iommu_passthrough ||
> > +         iommu_use_hap_pt(currd) || need_iommu(currd) )
> 
> I guess it's clearer to directly check iommu_dom0_strict here

Well, the problem with that is that it totally ties this interface to dom0. Whilst, in practice, that is the case at the moment (because of the xsm check) I do want to leave the potential to allow other PV domains to control their IOMMU mappings, if that make sense in future.

  Paul

> 
> > +        return false;
> > +
> > +    return true;
> > +}
> 

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