On 21 September 2015 at 09:44, David Woods [off-list ref] wrote:

Steve,

Hi Dave,

Thanks for your review and comments.  I take your points about the 16k
granule - it's helpful to know that support is in the works. However, I'm
not sure I agree with your reading of section 4.4.2. It's clear that for 16k
granules, the number of contiguous pages is different for the PTE and PMD
levels.  But I don't see anywhere it says that for 4K and 64K that the
contig bit is not supported at the PMD level - just that the number of
contiguous pages is the same at each level.

Many apologies, I appear to have led you down the garden path there.
Having double checked at ARM, the valid contiguous page sizes are indeed:
4K granule:
16 x ptes = 64K
16 x pmds = 32M
16 x puds = 16G

16K granule:
128 x ptes = 2M
32 x pmds = 1G

64K granule:
32 x ptes = 2M
32 x pmds = 16G

I tried using the tarmac trace module of the ARM simulator to support this
idea by turning on MMU tracing.  Using 4k granule, I created 64k and 32m
pages and touched each location in the page.  In both cases, the trace
recorded just one TLB fill (rather than the 16 you'd expect if the
contiguous bit were being ignored) and it indicated the expected page size.

1817498494 clk cpu2 TLB FILL cpu2.S1TLB 64K 0x2000000000_NS vmid=0, nG
asid=303:0x08fa360000_NS Normal InnerShareable Inner=WriteBackWriteAllocate
Outer=WriteBackWriteAllocate xn=0 pxn=1 ContiguousHint=1

1263366314 clk cpu2 TLB FILL cpu2.UTLB 32M 0x2000000000_NS vmid=0, nG
asid=300:0x08f6000000_NS Normal InnerShareable Inner=WriteBackWriteAllocate
Outer=WriteBackWriteAllocate xn=0 pxn=1 ContiguousHint=1

I'll try this with a 64k granule next.  I'm not sure what will happen with
16G pages since we are using an A53 model which I don't think supports such
large pages.

The Cortex-A53 supported TLB sizes can be found in the TRM:
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0500f/Chddiifa.html

My understanding is that the core is allowed to ignore the contiguous
bit if it doesn't support the particular TLB entry size, or substitute
in a slightly smaller TLB entry than hinted possible. Anyway, do give
it a go :-).

Cheers,
--
Steve