lkml.org 
[lkml]   [2013]   [Jun]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: ARM pagetable setting in Linux
[adding the ARM list -- please try and remember to do that in future]

On Wed, Jun 26, 2013 at 03:41:40AM +0100, Wang, Yalin wrote:
> Hi Will,

Hello,

> I have a question about arm pagetable setting in Linux .
>
> From armV6, there is TTBR0 and TTBR1 translation base address registers in mmu .
> But I found linux only use TTBR0 for translation base address ,
> Could we use TTBR0 and TTBR1 to split user task and kernel pagetables (swapper_pg_dir)?
>
> But I found this need set TTBCR.N ,
> If PAGE_OFFSET==0xc0000000 , set TTBCR.N=0x2 ,
> Will not work , because only ensure 0--0x00FFFFFF MVA use TTBCR0
> And other MVA will use TTBCR1 ,
> But if we set PAGE_OFFSET==0x80000000 (2GB split ) ,
> And set TTBCR.N=0x1 ,
> This will make sure user task MVA use TTBCR0 and kernel MVA
> Use TTBCR1 , this will make some improvement for the system .
>
> 1. Because we don’t need copy kernel first –level pagetables into every
> User task’s pagetables and flush tlb (for example fork() a new process).

Well, you still need the TLB maintenance for setting up CoW, so this win is
probably not very big.

> 2. And don’t need handle kernel page fault because that user task’s kernel
> Pagetable when it is not set up , need copy again( for example vmalloc() ioremap() kmap() will change
> Kernel pagetables and need update to every task pagetables ) .

Is that really a fastpath?

> 3. We even can only allocate 8KB first level pagetables for user task to save
> Memorys .

That would be a nice gain, but there are a number of factors here:

1. Pre-ARMv6 only have one ttbr, so you'd end up needing to support
both configurations in the same kernel source.

2. The module area lives below PAGE_OFFSET, so you still have to
deal with that correctly (mitigating some of your earlier points)

3. PAGE_OFFSET is not fixed at 2GB, so you constrain your use-case
even further.

4. We do actually use TTBR1 to hold an identity mapping at the
moment. This could probably be fixed, but would require some
thought.

There's undoubtedly other problems that haven't immediately come to me but,
all in all, I don't think this is a significant enough win to justify the
effort and maintenance headache.

Cheers,

Will
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2013-06-26 20:01    [W:1.765 / U:0.004 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site