Re: [PATCH] arm64: relax jump label ASM constraints

["Rémi Denis-Courmont" <remi@remlab.net>]

Hi,

Le keskiviikkona 4. maaliskuuta 2020, 14.39.33 EET Mark Rutland a écrit :
> On Wed, Mar 04, 2020 at 11:34:19AM +0200, Rémi Denis-Courmont wrote:
> > From: Remi Denis-Courmont <remi.denis.courmont@huawei.com>
> > 
> > The static key address is stored in the jump label table. It needs to
> > be a run-time constant. However, it does not need to be a constant
> > suitable for expansion as an immediate value, given that it is
> > expanded in a full 64-bits (.quad) statement.
> 
> I'm not sure that's quite true,

It it indeed not true. Some time elapsed between writing the patch and 
describing it, and I'd forgetten the exact details in the mean time :$

What's actually happening is that the current constraint wants a constant, but 
in reality, either a constant or a symbol is acceptable. In the later case, 
the .quad will be a constant.

> since it's used in an expression
> evaluated by the assembler. IIRC the "%c0 - ." expression cannot be
> represented by an AArch64 ELF relocation.

Changing the constraint won't affect the ability or inability of the compiler 
and linker to generate a relocation. In this case, the value is essentially an 
offset and it just works either way.

> Have I missed something that 'S' allows that 'i' does not, or is there a
> functional problem today?

No. I don't remember exactly which compiler or compiler flags and kernel 
configuration I was trying to notice the inconsistency. But it's only a latent 
issue as far as I am aware.

-- 
Rémi Denis-Courmont
http://www.remlab.net/




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