[RFC/RFT PATCH] arm64: mm: allow userland to run with one fewer translation level

[Ard Biesheuvel]

On 2 September 2016 at 17:58, Alexander Graf <agraf at suse.de> wrote:
>
>
> On 21.08.16 14:18, Ard Biesheuvel wrote:
>> The choice of VA size is usually decided by the requirements on the kernel
>> side, particularly the size of the linear region, which must be large
>> enough to cover all of physical memory, including the holes in between,
>> which may be very large (~512 GB on some systems).
>>
>> Since running with more translation levels could potentially result in
>> a performance penalty due to additional TLB pressure, this patch allows the
>> kernel to be configured so that it runs with one fewer translation level on
>> the userland side. Rather than modifying all the compile time logic to deal
>> with folded PUDs or PMDs, we simply allocate the root table and the next
>> table adjacently, so that we can simply point TTBR0_EL1 to the next table
>> (and update TCR_EL1.T0SZ accordingly)
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>
>> This is just a proof of concept. *If* there is a performance penalty associated
>> with using 4 translation levels instead of 3, I would expect this patch to
>> compensate for that, given that the additional TLB pressure should be on the
>> userland side primarily. Benchmark results are highly appreciated.
>>
>> As a bonus, this would fix the horrible yet real JIT issues we have been seeing
>> with 48-bit VA configurations. IOW, I expect this to be an easier sell than
>> simply limiting TASKSIZE to 47 bits (assuming anyone can show a benchmark where
>> this patch has a positive impact on the performance of a 48-bit/4 levels kernel)
>> and distros can ship kernels that work on all hardware (including Freescale and
>> Xgene with >= 64 GB) but don't break their JITs.
>>
>> This patch is most likely broken for 16k/47-bit configs, but I didn't bother to
>> fix that before having the discussion.
>
> Let's roll forward by a few years. In that time, there's a good chance
> you will have nvdimms in a good number of systems out there with massive
> address spaces that easily reach beyond the lousy 512GB you get with 3
> levels.
>

That still does not mean it makes sense for 48 bits to be the default
for every userland process.

> That means at that point we'd have to roll back and have 48 bits
> regardless - or add special attributes to have binaries that then can
> demand bigger address space. Overall that doesn't sound terribly
> appealing, so I'm not sure going for 39 as interim is a step into the
> right direction.
>
> That said, I'd be very happy to see benchmark results too :)
>

Well, my point is that there is no guaranteed minimum at the moment.
If you happen to be running on a 38-bit VA kernel, that is all you are
ever going to get. This means that either you deal with that, or you
need to signal in some way that 39-bit VA is insufficient.

The longer we leave the current undefined state endure, the more hacks
will come into existence (using munmap() etc) to make inferences about
what the current system provide. So we need to select something, stick
with it for now, and in the future, when it becomes necessary, expose
the means for a userland process to convey its minimum VA size.