Re: Making MAX_PHYS_ADDRESS_BITS configurable
From: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Date: Fri, 24 May 2019 20:51:33 +0200
> On 5/24/19 6:20 PM, David Miller wrote:
>>> On the other hand, arm64 currently allows the virtual address size
>>> to be configurable, currently defaulting to 48 bits [2, 3]. I was
>>> therefore wondering whether we could make MAX_PHYS_ADDRESS_BITS [4]
>>> configurable as well to be able to support these JITs on
>>> Debian/sparc64 for the foreseeable future by limiting the virtual
>>> address space to 47 or 48 bits.
>>
>> You can't just do this.
>>
>> It is possible that all physical memory is mapped to the top of the
>> mappable physical address range, therefore we really need to use the
>> full maximum setting supported by the CPU.
>
> Yes, my initial mail was incorrect. What I actually meant was reducing
> the size of the va_hole in userspace so that the top-most address that
> mmap() would return is not beyond 2^47.
>
> Would it be possible to add such a workaround until the JITs have fixed
> their broken code?
I suppose that's doable, sure.
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