Subject: [PATCH] arm64: mm: Create gigabyte kernel logical mappings where possible

From: [email-masked] (Steve Capper)
Date: Thu, 1 May 2014 09:54:12 +0100

On Wed, Apr 30, 2014 at 08:11:26PM +0200, Arnd Bergmann wrote:
> On Wednesday 30 April 2014 12:36:22 Steve Capper wrote:
> > We have the capability to map 1GB level 1 blocks when using a 4K
> > granule.
> > 
> > This patch adjusts the create_mapping logic s.t. when mapping physical
> > memory on boot, we attempt to use a 1GB block if both the VA and PA
> > start and end are 1GB aligned. This both reduces the levels of lookup
> > required to resolve a kernel logical address, as well as reduces TLB
> > pressure on cores that support 1GB TLB entries.
> > 
> > Signed-off-by: Steve Capper <[email-masked]>
> > ---
> > Hello,
> > This patch has been tested on the FastModel for 4K and 64K pages.
> > Also, this has been tested with Jungseok's 4 level patch.
> > 
> > I put in the explicit check for PAGE_SHIFT, as I am anticipating a
> > three level 64KB configuration at some point.
> > 
> > With two level 64K, a PUD is equivalent to a PMD which is equivalent to
> > a PGD, and these are all level 2 descriptors.
> > 
> > Under three level 64K, a PUD would be equivalent to a PGD which would
> > be a level 1 descriptor thus may not be a block.
> > 
> > Comments/critique/testers welcome.
> 
> It seems like a great idea. I have to admit that I don't understand
> the existing code, but what are the page sizes used here?

Actually, I think it was your idea ;-). I remember you talking about
increasing the mapping size when 4-level page tables were being
discussed. (I think I should have added a Reported-by, would be happy
to if you want?).

With a 64KB granule, we'll map 512MB blocks if possible, otherwise 64K.
And with a 4KB granule, the original code will map 2MB blocks if
possible, and 4KB otherwise.

The patch will make the 4KB granule case also map 1GB blocks if
possible.

> 
> Does the code always use the largest possible page size, or does
> it just use either small pages or 1G pages?

The code will put down the largest mappings it can. As the physical
memory sizes/address are very likely to be aligned to whatever block
size we use; we are likely to achieve the maximum size for our
mappings.

> 
> In combination with the contiguous page hint, we should be able
> to theoretically support 4KB/64KB/2M/32M/1G/16G TLBs in any
> combination for boot-time mappings on a 4K page size kernel,
> or 64KB/1M/512M/8G on a 64KB page size kernel.
> 

A contiguous hint could be applied to these mappings. The logic would
be a bit more complicated though when we consider different granules.
For 4KB we chain together 16 entries, for 64KB we use 32. If/when we
adopt a 16KB granule, we use 32 entries for a level 2 lookup and
128 entries for a level 3 lookup...

The largest TLB entry sizes that I am aware of in play are the block
sizes (i.e. 2MB, 512MB, 1GB). So I don't think we'll get any benefit at
the moment for adding the contiguous logic.

Cheers,
-- 
Steve

> 	Arnd