Re: [RFC, PATCHv1 00/28] 5-level paging

[Dave Hansen]

On 12/08/2016 09:01 PM, Ingo Molnar wrote:
>> >   - Handle opt-in wider address space for userspace.
>> > 
>> >     Not all userspace is ready to handle addresses wider than current
>> >     47-bits. At least some JIT compiler make use of upper bits to encode
>> >     their info.
>> > 
>> >     We need to have an interface to opt-in wider addresses from userspace
>> >     to avoid regressions.
>> > 
>> >     For now, I've included testing-only patch which bumps TASK_SIZE to
>> >     56-bits. This can be handy for testing to see what breaks if we max-out
>> >     size of virtual address space.
> So this is just a detail - but it sounds a bit limiting to me to provide an 'opt 
> in' flag for something that will work just fine on the vast majority of 64-bit 
> software.

MPX is going to be a real pain here.  It is relatively transparent to
applications that use it, and old MPX binaries are entirely incompatible
with the new address space size, so an opt-out wouldn't be friendly.

Because the top-level MPX bounds table is indexed by the virtual
address, a growth in vaddr space is going to require the table to grow
(or change somehow).  The solution baked into the hardware spec is to
just make the top-level table 512x larger to accommodate the 512x
increase in vaddr space.  (This behavior is controlled by a new MSR, btw...)

So, either we disable MPX on all old MPX binaries by returning an error
when the prctl() tries to enable MPX and 5-level paging is on, or we go
with some form of an opt-in.  New MPX binaries will opt-in to the larger
address space since they know to allocate the new, larger table.