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Re: [Xen-devel] [PATCH] xen/typesafe: Force helpers to be always_inline


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Tue, 1 Oct 2019 21:59:39 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, WeiLiu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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On 01/10/2019 09:38, Jan Beulich wrote:
> On 30.09.2019 21:16, Andrew Cooper wrote:
>> Clang in particular has a habit of out-of-lining these and creating multiple
>> local copies of _mfn() and mfn_x(), etc.  Override this behaviour.
> Is special casing the typesafe helpers then the right approach? The
> fundamental idea after all is to let the compiler decide. I certainly
> agree that not inlining such trivial functions despite the inline
> keyword looks far from optimal, but if there's such a general issue
> with clang, shouldn't we make "inline" expand to "always_inline"
> uniformly?

Inline handing is a mess.

We currently define inline to __inline__.  Undoing this results in build
failures.

Linux currently defines inline to always_inline and they are desperately
trying to undo this (mis)behaviour.

There are a few uses of always_inline for safety purposes (the
speculative helpers).  Most uses of always_inline look to be workarounds
for the size-of-asm bug/(mis)feature.

In an ideal world, we wouldn't need it at all, but I definitely don't
think that taking the Linux approach is a clever move.  We definitely
have some static inlines which would better not being inline.

~Andrew

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