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

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Message-ID: <CAObL_7FW7qr5cOpB=kcPaQQUgPAebm+WgzevTW8j83WKV3dFDQ@mail.gmail.com>
Date:	Tue, 21 Feb 2012 11:51:23 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Tue, Feb 21, 2012 at 11:49 AM, H. Peter Anvin <hpa@...or.com> 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.

--Andy
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