Re: [Xen-devel] Hyp compat_memory_op() and 256 GB PV

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

Keir Fraser wrote:
> On 18/02/2009 03:39, "Mukesh Rathor" <mukesh.rathor@xxxxxxxxxx> wrote:
>
>> Moving on to 256 GB guest, the hyp is failing the XENMEM_populate_physmap
>> hcall in compat_memory_op(). The problem is size too large for continuation
>> encoding:
>>
>>      /* Is size too large for us to encode a continuation? */
>>      if ( cmp.rsrv.nr_extents > (UINT_MAX >> MEMOP_EXTENT_SHIFT))
>>          return start_extent;
>>
>>       for 256 GB : nr_extents == 0x4000000
>>
>>   Currently at a loss on this one!
>
> Well, who's making the compat call? Not the guest itself presumably since it
> is 64-bit? So it's probably dom0? But I would think that dom0 would only do
> large amounts of allocation for the new domU in xc_hvm_build.c, and that is
> careful to allocate memory in batches of 8MB at a time.
>
> Basically you need to track down the call site of the failed
> compat_memory_op().
>
>  -- Keir
>

Hi Keir,

I see this in domain-builder-ng.log:

elf_xen_addr_calc_check: addresses:
    virt_base        = 0xffffffff80000000
    elf_paddr_offset = 0xffffffff80000000
    virt_offset      = 0x0
    virt_kstart      = 0xffffffff80200000
    virt_kend        = 0xffffffff807014e4
    virt_entry       = 0xffffffff80200000
xc_dom_parse_elf_kernel: xen-3.0-x86_64: 0xffffffff80200000 ->
0xffffffff807014e4
xc_dom_mem_init: mem 262144 MB, pages 0x4000000 pages, 4k each
xc_dom_mem_init: 0x4000000 pages
xc_dom_boot_mem_init: called
x86_compat: guest xen-3.0-x86_64, address size 64
xc_dom_malloc            : 256 MB
xc_dom_boot.c:143: panic: xc_dom_boot_mem_init: can't allocate low
memory for domain


I set a breakpoint in compat_memory_op() and stepping thru it, noticed
it was bailing out at the if statement, and nr_extents was 0x4000000.

This is a PV domain BTW, I guess xc_hvm_build.c wont' be involved. I'll
instrument libxc and debug more... meanwhile I thought I'd post this.

Thanks,
Mukesh

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