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

[Oleksandr <olekstysh@gmail.com>]

On 22.09.21 01:00, Stefano Stabellini wrote:

Hi Stefano

> On Tue, 21 Sep 2021, Oleksandr wrote:
>> On 21.09.21 02:21, Stefano Stabellini wrote:
>>> On Sun, 19 Sep 2021, Oleksandr wrote:
>>>>> On 18/09/2021 03:37, Stefano Stabellini wrote:
>>>>>> 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).
>>>>> This is rather annoying, but not unheard. There are a couple of
>>>>> platforms
>>>>> where the MMIOs are not fully described in the DT.
>>>>>
>>>>> In fact, we have a callback 'specific_mappings' which create additional
>>>>> mappings (e.g. on the omap5) for dom0.
>>>>>
>>>>>> 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.
>>>>> (Oleksandr, it looks like some rationale about the different approach is
>>>>> missing in the commit message. Can you add it?)
>>>> Yes sure, but let me please clarify what is different approach in this
>>>> context. Is it to *also* take into the account MMIO regions of the devices
>>>> for
>>>> passthrough for case when IOMMU is off (in addition to unallocated
>>>> memory)? If
>>>> yes, I wonder whether we will gain much with that according to that
>>>> device's
>>>> MMIO regions are usually not big enough and we stick to allocate extended
>>>> regions with bigger size (> 64MB).
>>> That's fair enough. There are a couple of counter examples where the
>>> MMIO regions for the device to assign are quite large, for instance a
>>> GPU, Xilinx AIEngine, or the PCIe Root Complex with the entire aperture,
>>> but maybe they are not that common. I am not sure if it is worth
>>> scanning the tree for xen,passthrough regions every time at boot for
>>> this.
>> ok, I will add a few sentences to commit message about this different approach
>> for now. At least this could be implemented later on if there is a need.
> One thing that worries me about this is that if we take an old Xen with
> this series and run it on a new board, it might cause problems. At the
> very least [1] wouldn't work.

I got it.


>
> Can we have a Xen command line argument to disable extended regions as
> an emergecy toggle?

I think, yes. If no preference for the argument name I will name it 
"no-ext-region".

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

-- 
Regards,

Oleksandr Tyshchenko