Re: [Xen-devel] [PATCH for-4.9] xen/arm: acpi: Map MMIO on fault in stage-2 page table for the hardware domain

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 30 Mar 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 30/03/17 19:37, Stefano Stabellini wrote:
> > On Thu, 30 Mar 2017, Julien Grall wrote:
> > > +/*
> > > + * When using ACPI, most of the MMIO regions will be mapped on-demand
> > > + * in stage-2 page tables for the hardware domain because Xen is not
> > > + * able to know from the EFI memory map the MMIO regions.
> > > + */
> > > +static bool try_map_mmio(gfn_t gfn)
> > > +{
> > > +    struct domain *d = current->domain;
> > > +
> > > +    /* For the hardware domain, all MMIOs are mapped with GFN == MFN */
> > > +    mfn_t mfn = _mfn(gfn_x(gfn));
> > > +
> > > +    /*
> > > +     * Device-Tree should already have everything mapped when building
> > > +     * the hardware domain.
> > > +     */
> > > +    if ( acpi_disabled )
> > > +        return false;
> > > +
> > > +    if ( d != hardware_domain )
> > 
> > is_hardware_domain(d)
> 
> Will fix it.
> 
> > 
> > > +        return false;
> > > +
> > > +    /* The hardware domain can only map permitted MMIO regions */
> > > +    if ( !iomem_access_permitted(d, mfn_x(mfn), mfn_x(mfn) + 1) )
> > > +        return false;
> > 
> > Because of the potential of causing problems with DMA to device MMIO,
> > I would add a warning here, something like:
> > 
> >    dprintk(XENLOG_WARNING, "Mapping gfn=mfn=%"PRI_gfn" to Dom%d\n",
> > d->domain_id);
> > 
> > I hope that it will help us debug issues when/if we'll actually
> > encounter such a case.
> 
> I don't think this will help us and it will flood the console in debug-build
> (there are a lot of MMIO to map).
> 
> When SMMUs are not used, DMA to device MMIO will work regardless the mapping
> in stage-2.
> 
> When SMMUs are in use, you will get a SMMU fault that tells you the address
> used. With that you could know the region was not mapped.
 
Fair enough. Do you think there is any value in adding only one debug
warning, to say that we are mapping MMIOs on trap on this platform? But
today, it would basically be the same as saying "ACPI is enabled".

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