Hi Jan,
On 17/01/17 09:09, Jan Beulich wrote:
On 16.01.17 at 19:46, <julien.gr...@arm.com> wrote:
On 09/01/17 08:40, Jan Beulich wrote:
On 07.01.17 at 07:05, <vijay.kil...@gmail.com> 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"?
Yes sorry.
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.
DIRECTMAP_VIRT_END is defined because all the virtual region defined are
bound using 2 defines (*_VIRT_START and *_VIRT_END). Removing
DIRECTMAP_VIRT_END would be the worst solution because it only defers
the problem until we decide to not map all the memory in Xen for ARM64.
So I would prefer to see an arch_* helper here.
Regards,
--
Julien Grall
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