Re: [Xen-devel] arm: Boot allocator fails with multi node memory

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.01.17 at 19:46, <julien.grall@xxxxxxx> wrote:
> On 09/01/17 08:40, Jan Beulich wrote:
>>>>> On 07.01.17 at 07:05, <vijay.kilari@xxxxxxxxx> wrote:
>>> Question: Why this address is not mapped?. If mapped where this va is
>>> mapped?.
>>
>> Well, I think this is the wrong question to ask. Why would it be mapped
>> if there's no memory there?
>>
>>> (XEN) Walking Hypervisor VA 0x847fffffffff on CPU0 via TTBR
>>> 0x00000000ffcf8000
>>> (XEN) 0TH[0x108] = 0x0000000000000000
>>>
>>>
>>> static unsigned long init_node_heap(int node, unsigned long mfn,
>>>                                     unsigned long nr, bool_t *use_tail, int 
>>> d)
>>> {
>>> #ifdef DIRECTMAP_VIRT_END
>>>     unsigned long eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END);
>>> #endif
>>>
>>>
>>> ....
>>>
>>>     else if ( nr >= needed &&
>>>               (mfn + needed) <= (virt_to_mfn(eva - 1) + 1) &&  // <===
>>> FAILS here
>>
>> The assumption here is that a virtual address inside the direct map
>> can always be translated, and from your report I'm gaining the
>> understanding that this is simply not true on ARM (but the code
>> here pre-dates ARM iirc). If that assumption doesn't hold (and
>> cannot be made so), apart from adjusting the code here there may
>> need to be a full audit of common code to see whether there are\
>> any other such uses.
> 
> On ARM, the function virt_to_mfn uses the hardware to do the address 
> translation. So if the virtual address is not mapped, it will fail.
> 
> This is different from the assumption made by the code and x86
> behavior. I'd still like to keep the current behavior on ARM as it is 
> very handy to directly get debug information when the virtual address is 
> not mapped. Stefano, do you have any opinions?
> 
> Looking at the code pointed out by Vijay. If I understand correctly, 
> virt_to_mfn(eva) is used to find out the maximum MFN that can be mapped.

Right.

> On ARM64, all the memory is direct mapped so far, so this check will 
> always be false.

DYM "will always be true"?

> This might not be true in the future.

Right. But regardless of that we clearly need another arch_*()
abstraction here, which (for now) returns constant true on ARM.

Or wait - why does ARM have DIRECTMAP_VIRT_END defined? I
can't see any use of it outside of page_alloc.c, and there all the
problematic code would be compiled out if the symbol wasn't
defined.

Jan


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