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

["Jan Beulich" <JBeulich@xxxxxxxx>]

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.

01: VMX: reduce number of posted-interrupt hooks
02: VMX: don't unconditionally set the tsc_scaling.setup hook
03: x86/HVM: switch virtual_intr_delivery_enabled() hook to simple boolean
04: x86/HVM: drop vmfunc_intercept
05: x86/HVM: add wrapper for hvm_funcs.set_tsc_offset()
06: x86: allow producing .i or .s for multiply compiled files
07: x86/shadow: fetch CPL just once in sh_page_fault()
08: x86/alternatives: allow using assembler macros in favor of C ones
09: x86: infrastructure to allow converting certain indirect calls to direct 
ones
10: x86/HVM: patch indirect calls through hvm_funcs to direct ones
11: x86/HVM: patch vINTR indirect calls through hvm_funcs to direct ones
12: x86: patch ctxt_switch_masking() indirect call to direct one
13: x86/genapic: drop .target_cpus() hook
14: x86/genapic: remove indirection from genapic hook accesses
15: x86/genapic: patch indirect calls to direct ones
16: x86/cpuidle: patch some indirect calls to direct ones

Jan



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