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Re: [PATCH 30/30] x32: Add x32 VDSO support

On Tue, Feb 21, 2012 at 11:49 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> On 02/21/2012 11:40 AM, Andrew Lutomirski wrote:
>>> I was having delusions that we could have a task-owned PDT in negative
>>> space, but that would require unsharing the third level, too, which is
>>> just way too messy.
>> I'd like to do that, too, and I'd also like to have a per-cpu
>> kernel-only page in there, but that's even worse.  If we had a
>> separate cr3-like register for negative addresses, life would be good
>> :)
> No, that wouldn't help.  The situation is actually quite similar to the
> current situation where we have an unshared fourth level, but since the
> fourth entries are 512G per entry, we would have to push unsharing of
> the kernel address space at least one more level (1G), possibly two
> (2M).  Painful.
> The main advantage of a separate cr3 would be that we wouldn't need the
> unshared top level for the kernel side.

Also, as is, if the top level wants to be per-cpu *and* per-task,
that's a big explosion of page tables that all need to stay in sync.

Oh well.

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