Re: [Xen-devel] [PATCH 00/16] x86: indirect call overhead reduction

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.07.18 at 15:15, <JBeulich@xxxxxxxx> wrote:
> While indirect calls have always been more expensive than direct ones,
> their cost has further increased with the Spectre v2 mitigations. In a
> number of cases we simply pointlessly use them in the first place. In
> many other cases the indirection solely exists to abstract from e.g.
> vendor specific hardware details, and hence the pointers used never
> change once set. Here we can use alternatives patching to get rid of
> the indirection.
> 
> From patch 8 onwards dependencies exist on earlier, yet to be reviewed
> patches ("x86/alternatives: fully leverage automatic NOP filling" as well
> as the "x86: improve PDX <-> PFN and alike translations" series at the
> very least). I nevertheless wanted to enable a first round of review of
> the series, the more that some of the patches (not just initial ones)
> could perhaps be taken irrespective of those dependencies.
> 
> Further areas where indirect calls could be eliminated (and that I've put
> on my todo list in case the general concept here is deemed reasonable)
> are IOMMU, cpufreq, vPMU, and XSM. For some of these, the ARM side
> would need dealing with as well - I'm not sure whether replacing indirect
> calls by direct ones is worthwhile there as well; if not, the wrappers
> would simply need to become function invocations in the ARM case.

Btw, I didn't want to Cc you on the whole series, but input on the above
would certainly be helpful to decide how to deal with indirect calls in code
shared between the architectures.

Thanks, Jan



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