linux-kernel - Re: [PATCH 30/30] x32: Add x32 VDSO support

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Message-ID: <4F43F54D.50201@zytor.com>
Date:	Tue, 21 Feb 2012 11:49:33 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <luto@....edu>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	mingo@...nel.org, tglx@...utronix.de,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	hjl.tools@...il.com
Subject: Re: [PATCH 30/30] x32: Add x32 VDSO support

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.

	-hpa
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