[PATCH v2 9/9] arm64: Work around systems with mismatched cache line sizes

On Fri, Aug 26, 2016 at 05:16:27PM +0100, Will Deacon wrote:

On Fri, Aug 26, 2016 at 02:08:01PM +0100, Suzuki K Poulose wrote:

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On 26/08/16 14:04, Suzuki K Poulose wrote:

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On 26/08/16 12:03, Ard Biesheuvel wrote:

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IMO, this is a pattern that we should avoid: you are introducing one
instance now, which will make it hard to say no to the next one in the
future. Isn't there a better way to organize the arm64_ftr_reg array
that allows us to reference entries directly? Ideally, a way that gets
rid of the runtime sorting, since I don't think that is a good
replacement for developer discipline anyway (although I should have
spoken up when that was first introduced) Or am I missing something
here?

I'm not sure we can have some simple direct access. Suzuki did some
grouping originally but I wouldn't call that a direct access either,
more like iterating through several groups.

A possibility would be to generate global variables for each
arm64_ftr_reg with the ARM64_FTR_REG macro extended to also place a
pointer in a dedicated section to be used as array. Assembly code would
access the global variable directly. But I'm not really sure it's worth
it.

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I had some form of direct access to the feature register in one of
the versions [0], but was dropped based on Catalin's suggestion at [1].

Forgot to add, [0] wouldn't solve this issue cleanly either. It would simply
speed up the read_system_reg(). So we do need a call to read_system_reg()
from assembly code, which makes it a little bit tricky.

It might be worth looking to see if we can pass the ctr as an extra
parameter to the assembly routines that need it. Then you can access it
easily from C code, and if you pass it as 0 that could result in the asm
code reading it from the h/w register, removing the need for the _raw
stuff you add.

How often to we need to access a sanitised sysreg from assembly? AFAICT,
CTR_EL0 is the first. If we only need it to infer the minimum cache line
size, we could as well store the latter in a global variable and access
it directly. If we feel brave, we could patch a "mov \reg, #x"
instruction in the ?cache_line_size macros (starting with 32 by default,
though to make it less cumbersome we'd have to improve the run-time
patching code a bit).

-- 
Catalin