Re: [Xen-devel] [PATCH v13 0/4] add per-domain IOMMU control

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Julien Grall <Julien.Grall@xxxxxxx>
> Sent: 25 September 2019 22:34
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Oleksandr' 
> <olekstysh@xxxxxxxxx>; 'Jan Beulich'
> <jbeulich@xxxxxxxx>
> Cc: nd <nd@xxxxxxx>; Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>; Stefano 
> Stabellini
> <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; KonradRzeszutek Wilk 
> <konrad.wilk@xxxxxxxxxx>; Andrew
> Cooper <Andrew.Cooper3@xxxxxxxxxx>; David Scott <dave@xxxxxxxxxx>; Tim 
> (Xen.org) <tim@xxxxxxx>; George
> Dunlap <George.Dunlap@xxxxxxxxxx>; Tamas K Lengyel <tamas@xxxxxxxxxxxxx>; Ian 
> Jackson
> <Ian.Jackson@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx;
> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monne 
> <roger.pau@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH v13 0/4] add per-domain IOMMU control
> 
> Hi,
> 
> On 25/09/2019 17:10, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Oleksandr <olekstysh@xxxxxxxxx>
> >> Sent: 25 September 2019 16:50
> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich' 
> >> <jbeulich@xxxxxxxx>
> >> Cc: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>; Stefano Stabellini 
> >> <sstabellini@xxxxxxxxxx>; Wei
> Liu
> >> <wl@xxxxxxx>; KonradRzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Andrew Cooper
> >> <Andrew.Cooper3@xxxxxxxxxx>; David Scott <dave@xxxxxxxxxx>; Tim (Xen.org) 
> >> <tim@xxxxxxx>; George
> Dunlap
> >> <George.Dunlap@xxxxxxxxxx>; Tamas K Lengyel <tamas@xxxxxxxxxxxxx>; Ian 
> >> Jackson
> >> <Ian.Jackson@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx;
> >> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Roger Pau Monne 
> >> <roger.pau@xxxxxxxxxx>; Julien
> Grall
> >> <julien.grall@xxxxxxx>
> >> Subject: Re: [Xen-devel] [PATCH v13 0/4] add per-domain IOMMU control
> >>
> >>
> >> [CC Julien]
> >>
> >>
> >> Hi Paul
> >>
> >> I may mistake, but looks like
> >>
> >> 80ff3d338dc93260b41ffeeebb0f852c2edef9ce iommu: tidy up
> >> iommu_use_hap_pt() and need_iommu_pt_sync() macros
> >>
> >> triggers ASSERT_UNREACHABLE on Arm if no IOMMU has been found (I built
> >> with my platform's IOMMU driver disabled: # CONFIG_IPMMU_VMSA is not set) .
> >>
> >> So, iommu_setup() calls clear_iommu_hap_pt_share() with
> >> iommu_hap_pt_share being set (CONFIG_IOMMU_FORCE_PT_SHARE=y) which,
> >> actually, triggers ASSERT.
> >>
> >
> > Here a minimal patch, leaving 'force pt share' in place. Does this avoid 
> > the problem?
> >
> > ---8<---
> > diff --git a/xen/common/sysctl.c b/xen/common/sysctl.c
> > index e8763c7fdf..f88a285e7f 100644
> > --- a/xen/common/sysctl.c
> > +++ b/xen/common/sysctl.c
> > @@ -268,9 +268,11 @@ long do_sysctl(XEN_GUEST_HANDLE_PARAM(xen_sysctl_t) 
> > u_sysctl)
> >           pi->max_mfn = get_upper_mfn_bound();
> >           arch_do_physinfo(pi);
> >           if ( iommu_enabled )
> > +        {
> >               pi->capabilities |= XEN_SYSCTL_PHYSCAP_directio;
> > -        if ( iommu_hap_pt_share )
> > -            pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
> > +            if ( iommu_hap_pt_share )
> > +                pi->capabilities |= XEN_SYSCTL_PHYSCAP_iommu_hap_pt_share;
> > +        }
> >
> >           if ( copy_to_guest(u_sysctl, op, 1) )
> >               ret = -EFAULT;
> > diff --git a/xen/include/xen/iommu.h b/xen/include/xen/iommu.h
> > index 7c3003f3f1..6a10a24128 100644
> > --- a/xen/include/xen/iommu.h
> > +++ b/xen/include/xen/iommu.h
> > @@ -68,8 +68,6 @@ static inline void clear_iommu_hap_pt_share(void)
> >   {
> >   #ifndef iommu_hap_pt_share
> >       iommu_hap_pt_share = false;
> > -#elif iommu_hap_pt_share
> > -    ASSERT_UNREACHABLE();
> >   #endif
> 
> IHMO, calling this function is a mistake on platform only supporting
> shared page-table so the ASSERT() should be kept here.
> 
> This raises the question why the function is actually called from common
> code. iommu_hap_enabled() should technically not be used in any code if
> the IOMMU is not enabled/present. So what are you trying to prevent here?

What I'm trying to prevent, on x86, is a situation where the iommu_enabled == 
false but iommu_hap_pt_share == true. I had, mistakenly, believed that 
iommu_enabled would never be false for ARM but it seems this is not the case so 
that situation has to be tolerated. I guess, given the other hunk of my patch, 
I can actually leave the ASSERT in place and avoid making the call from common 
code, in which case the function ought to move into an x86 header as well.

  Paul

> 
> Cheers,
> 
> --
> Julien Grall
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