On 09/07/13 15:13, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 09, 2013 at 02:38:53PM +0100, David Vrabel wrote:
>> From: David Vrabel <david.vrabel@xxxxxxxxxx>
>>
>> If there are UNUSABLE regions in the machine memory map, dom0 will
>> attempt to map them 1:1 which is not permitted by Xen and the kernel
>> will crash.
>>
>> There isn't anything interesting in the UNUSABLE region that the dom0
>> kernel needs access to so we can avoid making the 1:1 mapping and
>> leave the region as RAM.
>>
>> Since the obtaining the memory map for dom0 and domU are now more
>> different, refactor each into separate functions.
>>
>> This fixes a dom0 boot failure if tboot is used (because tboot was
>> marking a memory region as UNUSABLE).
>
> Please also include the serial log that shows the crash.
It's a domain crash by Xen and it isn't useful as none of the stack is
decoded.
>> +static int __init xen_get_memory_map_dom0(struct e820entry *map,
>> + unsigned *nr_entries)
>> +{
>> + struct xen_memory_map memmap;
>> + unsigned i;
>> + int ret;
>> +
>> + /*
>> + * Dom0 requires access to machine addresses for BIOS data and
>> + * MMIO (e.g. PCI) devices. The reset of the kernel expects
>> + * to be able to access these through a 1:1 p2m mapping.
>> + *
>> + * We need to take the pseudo physical memory map and set up
>> + * 1:1 mappings corresponding to the RESERVED regions and
>> + * holes in the /machine/ memory map, adding/expanding the RAM
>> + * region at the end of the map for the relocated RAM.
This is the key paragraph. The apparent use of the machine memory map
for dom0 is a confusing fiction.
>> + *
>> + * This is more easily done if we just start with the machine
>> + * memory map.
>> + *
>> + * UNUSABLE regions are awkward, they are not interesting to
>> + * dom0 and Xen won't allow them to be mapped so we want to
>> + * leave these as RAM in the pseudo physical map.
>
> We just want them as such in the P2M but not do any PTE creation for it?
> Why do we care about it? We are not creating any page tables for
> E820_UNUSABLE regions.
I don't follow what you're asking here.
In dom0, UNUSABLE regions in the machine memory map are RAM regions on
the pseudo-physical memory map. So instead of playing games and making
these regions special in the pseudo-physical map we just leave them as RAM.
>> + *
>> + * Again, this is easiest if we take the machine memory map
>> + * and change the UNUSABLE regions to RAM.
>
> Won't then Linux try to map them then? In 3.9 (or was it 3.8?) and further
> the page table creation starts ignoring any E820 entries that are not RAM.
> See init_range_memory_mapping and its comment:
Yes. They are just regular RAM in the pseudo-physical map.
> /*
> * We need to iterate through the E820 memory map and create direct mappings
> * for only E820_RAM and E820_KERN_RESERVED regions. We cannot simply
> * create direct mappings for all pfns from [0 to max_low_pfn) and
> * [4GB to max_pfn) because of possible memory holes in high addresses
> * that cannot be marked as UC by fixed/variable range MTRRs.
> * Depending on the alignment of E820 ranges, this may possibly result
> * in using smaller size (i.e. 4K instead of 2M or 1G) page tables.
> *
>
>
> So in effect you are now altering them.
No.
>> + */
>> +
>> + memmap.nr_entries = *nr_entries;
>> + set_xen_guest_handle(memmap.buffer, map);
>> +
>> + ret = HYPERVISOR_memory_op(XENMEM_machine_memory_map, &memmap);
>> + if (ret < 0)
>> + return ret;
>> +
>> + for (i = 0; i < memmap.nr_entries; i++) {
>> + if (map[i].type == E820_UNUSABLE)
>
> What if the E820_UNUSABLE regions were manufactured by the BIOS? Or
> somebody booted Xen with mem=3G (in which we clip the memory) on a 16GB
> box.
The resulting memory map should be clipped by the result of the call to
xen_get_max_pages().
David
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