Re: [Xen-devel] [RFC 00/10] xen/arm: Implement the hypercall handling in assembly

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-12-15 at 18:33 +0000, Andrew Cooper wrote:
> On 15/12/15 17:51, Julien Grall wrote:
> > Hi all,
> > 
> > This patch aims is a first attempt to write the hypercall dispatch in
> > assembly
> > in order to avoid reloading the hypercall arguments from the stack.
> 
> Why?ÂÂA couple of stack operations are in the noise, performance wise.

At least on ARM It's quite a bit more than a couple (dozens but not
hundreds I would say, and I've not looked at the patch yet to see how much
it saves out of those), but clearly it would need to be measured at some
point before taking it too much further than an RFC.

There's a second issue which is that the current C dispatch table approach
which uses a common typedef is not strictly speaking valid C, since it is
undefined behaviour to call a function through a typedef which has a
different prototype (Ian J can probably explain it better than I, with
chapter and verse etc).

Ian.

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