Re: [Xen-devel] [PATCH for-4.12] xen/iommu: fix dev assignment on ARM after 91d4eca7

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 4 Jan 2019, Paul Durrant wrote:
> > -----Original Message-----
> > From: Stefano Stabellini [mailto:sstabellini@xxxxxxxxxx]
> > Sent: 03 January 2019 18:20
> > To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxx;
> > julien.grall@xxxxxxx; jbeulich@xxxxxxxx; Paul Durrant
> > <Paul.Durrant@xxxxxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH for-4.12] xen/iommu: fix dev assignment on
> > ARM after 91d4eca7
> > 
> > On Thu, 3 Jan 2019, Andrew Cooper wrote:
> > > On 03/01/2019 00:28, Stefano Stabellini wrote:
> > > > Fix device assignment on ARM after 91d4eca7 "mm / iommu: split
> > > > need_iommu() into has_iommu_pt() and need_iommu_pt_sync()".
> > > >
> > > > arch_iommu_populate_page_table returns -ENOSYS which causes
> > > > iommu_construct to return early, although it is not an error.
> > > >
> > > > Interestingly, the if ( rc ) was present even before 91d4eca7, but it
> > was
> > > > still working before. Now, with the new hd->status field it won't
> > > > complete the initialization successfully.
> > > >
> > > > Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> > > >
> > > > diff --git a/xen/drivers/passthrough/iommu.c
> > b/xen/drivers/passthrough/iommu.c
> > > > index ac62d7f..a63329b 100644
> > > > --- a/xen/drivers/passthrough/iommu.c
> > > > +++ b/xen/drivers/passthrough/iommu.c
> > > > @@ -269,7 +269,7 @@ int iommu_construct(struct domain *d)
> > > >          hd->need_sync = true;
> > > >
> > > >          rc = arch_iommu_populate_page_table(d);
> > >
> > > The comment in ARM's arch_iommu_populate_page_table() says
> > >
> > > /* The IOMMU shares the p2m with the CPU */
> > >
> > > Which means that iommu_use_hap_pt() (just out of context above this
> > > hunk) is wrong.  It should return true, which will prevent us entering
> > > this path during initialisation.
> > 
> > iommu_use_hap_pt is implemented by calling has_iommu_pt(d) (also on x86
> > is part of the implementation), which is implemented as:
> > 
> >   (dom_iommu(d)->status != IOMMU_STATUS_disabled)
> > 
> > IOMMU_STATUS_disabled is zero in the enum, so status starts as
> > IOMMU_STATUS_disabled. It is set to IOMMU_STATUS_initializing for dom0
> > in iommu_hwdom_init, but it is not set to IOMMU_STATUS_initializing for
> > other domains anywhere, leading to this error.
> 
> It is set for other domains... see iommu_construct().

Hi Paul, unfortunately it doesn't work that way. Let me explain:


iommu_construct() sets hd->status to IOMMU_STATUS_initializing *after*
calling iommu_use_hap_pt(d), and specifically inside the if statement:

    if ( !iommu_use_hap_pt(d) )
    {
        int rc;

        hd->status = IOMMU_STATUS_initializing;
        hd->need_sync = true;

        rc = arch_iommu_populate_page_table(d);
        if ( rc )
        {
            if ( rc != -ERESTART )
            {
                hd->need_sync = false;
                hd->status = IOMMU_STATUS_disabled;
            }

            return rc;
        }
    }

However, for iommu_use_hap_pt(d) to return the correct value, which
should be always true on ARM, status needs to be already set to
IOMMU_STATUS_initializing.

Because when we enter iommu_construct status is IOMMU_STATUS_disabled,
iommu_use_hap_pt(d) calls has_iommu_pt(d) which wrongly returns false
because it is implemented as:

#define has_iommu_pt(d) (dom_iommu(d)->status != IOMMU_STATUS_disabled)


I think we need to move the initialization of status to
IOMMU_STATUS_initializing earlier, specifically to iommu_domain_init.

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