Re: [PATCH V2 2/3] xen/arm: Add handling of extended regions for Dom0

[Stefano Stabellini <sstabellini@kernel.org>]

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On Fri, 17 Sep 2021, Stefano Stabellini wrote:
> On Fri, 17 Sep 2021, Oleksandr wrote:
> > > > +
> > > > +    dt_dprintk("Find unallocated memory for extended regions\n");
> > > > +
> > > > +    unalloc_mem = rangeset_new(NULL, NULL, 0);
> > > > +    if ( !unalloc_mem )
> > > > +        return -ENOMEM;
> > > > +
> > > > +    /* Start with all available RAM */
> > > > +    for ( i = 0; i < bootinfo.mem.nr_banks; i++ )
> > > > +    {
> > > > +        start = bootinfo.mem.bank[i].start;
> > > > +        end = bootinfo.mem.bank[i].start + bootinfo.mem.bank[i].size - 1;
> > > > +        res = rangeset_add_range(unalloc_mem, start, end);
> > > > +        if ( res )
> > > > +        {
> > > > +            printk(XENLOG_ERR "Failed to add: %#"PRIx64"->%#"PRIx64"\n",
> > > > +                   start, end);
> > > > +            goto out;
> > > > +        }
> > > > +    }
> > > > +
> > > > +    /* Remove RAM assigned to Dom0 */
> > > > +    for ( i = 0; i < assign_mem->nr_banks; i++ )
> > > > +    {
> > > > +        start = assign_mem->bank[i].start;
> > > > +        end = assign_mem->bank[i].start + assign_mem->bank[i].size - 1;
> > > > +        res = rangeset_remove_range(unalloc_mem, start, end);
> > > > +        if ( res )
> > > > +        {
> > > > +            printk(XENLOG_ERR "Failed to remove:
> > > > %#"PRIx64"->%#"PRIx64"\n",
> > > > +                   start, end);
> > > > +            goto out;
> > > > +        }
> > > > +    }
> > > > +
> > > > +    /* Remove reserved-memory regions */
> > > > +    for ( i = 0; i < bootinfo.reserved_mem.nr_banks; i++ )
> > > > +    {
> > > > +        start = bootinfo.reserved_mem.bank[i].start;
> > > > +        end = bootinfo.reserved_mem.bank[i].start +
> > > > +            bootinfo.reserved_mem.bank[i].size - 1;
> > > > +        res = rangeset_remove_range(unalloc_mem, start, end);
> > > > +        if ( res )
> > > > +        {
> > > > +            printk(XENLOG_ERR "Failed to remove:
> > > > %#"PRIx64"->%#"PRIx64"\n",
> > > > +                   start, end);
> > > > +            goto out;
> > > > +        }
> > > > +    }
> > > > +
> > > > +    /* Remove grant table region */
> > > > +    start = kinfo->gnttab_start;
> > > > +    end = kinfo->gnttab_start + kinfo->gnttab_size - 1;
> > > > +    res = rangeset_remove_range(unalloc_mem, start, end);
> > > > +    if ( res )
> > > > +    {
> > > > +        printk(XENLOG_ERR "Failed to remove: %#"PRIx64"->%#"PRIx64"\n",
> > > > +               start, end);
> > > > +        goto out;
> > > > +    }
> > > > +
> > > > +    start = EXT_REGION_START;
> > > > +    end = min((1ULL << p2m_ipa_bits) - 1, EXT_REGION_END);
> > > > +    res = rangeset_report_ranges(unalloc_mem, start, end,
> > > > +                                 add_ext_regions, ext_regions);
> > > > +    if ( res )
> > > > +        ext_regions->nr_banks = 0;
> > > > +    else if ( !ext_regions->nr_banks )
> > > > +        res = -ENOENT;
> > > > +
> > > > +out:
> > > > +    rangeset_destroy(unalloc_mem);
> > > > +
> > > > +    return res;
> > > > +}
> > > > +
> > > > +static int __init find_memory_holes(const struct kernel_info *kinfo,
> > > > +                                    struct meminfo *ext_regions)
> > > > +{
> > > > +    struct dt_device_node *np;
> > > > +    struct rangeset *mem_holes;
> > > > +    paddr_t start, end;
> > > > +    unsigned int i;
> > > > +    int res;
> > > > +
> > > > +    dt_dprintk("Find memory holes for extended regions\n");
> > > > +
> > > > +    mem_holes = rangeset_new(NULL, NULL, 0);
> > > > +    if ( !mem_holes )
> > > > +        return -ENOMEM;
> > > > +
> > > > +    /* Start with maximum possible addressable physical memory range */
> > > > +    start = EXT_REGION_START;
> > > > +    end = min((1ULL << p2m_ipa_bits) - 1, EXT_REGION_END);
> > > > +    res = rangeset_add_range(mem_holes, start, end);
> > > > +    if ( res )
> > > > +    {
> > > > +        printk(XENLOG_ERR "Failed to add: %#"PRIx64"->%#"PRIx64"\n",
> > > > +               start, end);
> > > > +        goto out;
> > > > +    }
> > > > +
> > > > +    /* Remove all regions described by "reg" property (MMIO, RAM, etc) */
> > > 
> > > Well... The loop below is not going to handle all the regions described in
> > > the property "reg". Instead, it will cover a subset of "reg" where the
> > > memory is addressable.
> > 
> > As I understand, we are only interested in subset of "reg" where the memory is
> > addressable.
> > 
> > 
> > > 
> > > 
> > > You will also need to cover "ranges" that will describe the BARs for the PCI
> > > devices.
> > Good point.
> 
> Yes, very good point!
> 
> 
> > Could you please clarify how to recognize whether it is a PCI
> > device as long as PCI support is not merged? Or just to find any device nodes
> > with non-empty "ranges" property
> > and retrieve addresses?
> 
> Normally any bus can have a ranges property with the aperture and
> possible address translations, including /amba (compatible =
> "simple-bus"). However, in these cases dt_device_get_address already
> takes care of it, see xen/common/device_tree.c:dt_device_get_address.
> 
> The PCI bus is special for 2 reasons:
> - the ranges property has a different format
> - the bus is hot-pluggable
> 
> So I think the only one that we need to treat specially is PCI.
> 
> As far as I am aware PCI is the only bus (or maybe just the only bus
> that we support?) where ranges means the aperture.

Now that I think about this, there is another "hotpluggable" scenario we
need to think about:

[1] https://marc.info/?l=xen-devel&m=163056546214978

Xilinx devices have FPGA regions with apertures currently not described
in device tree, where things can programmed in PL at runtime making new
devices appear with new MMIO regions out of thin air.

Now let me start by saying that yes, the entire programmable region
aperture could probably be described in device tree, however, in
reality it is not currently done in any of the device trees we use
(including the upstream device trees in linux.git).

So, we have a problem :-(


I can work toward getting the right info on device tree, but in reality
that is going to take time and for now the device tree doesn't have the
FPGA aperture in it. So if we accept this series as is, it is going to
stop features like [1] from working.

If we cannot come up with any better plans, I think it would be better
to drop find_memory_holes, only rely on find_unallocated_memory even
when the IOMMU is on. One idea is that we could add on top of the
regions found by find_unallocated_memory any MMIO regions marked as
xen,passthrough: they are safe because they are not going to dom0 anyway.

The only alternative I can think of is to have a per-board
enable/disable toggle for the extend region but it would be very ugly.