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Re: [Xen-devel] [PATCH] x86/boot: Simplify %fs setup in trampoline_setup

On 12.08.2019 17:10, Andrew Cooper wrote:
mov/shr is easier to follow than shld, and doesn't have a merge dependency on
the previous value of %edx.  Shorten the rest of the code by streamlining the

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

In addition to being clearer to follow, mov/shr is faster than shld to decode
and execute.  See https://godbolt.org/z/A5kvuC for the latency/throughput/port
analysis, the Intel Optimisation guide which classifes them as "Slow Int"
instructions, or the AMD Optimisation guide which specifically has a section
entitled "Alternatives to SHLD Instruction".

I don't really mind the change, but I don't think performance is a
concern here. Instead I think we want to size-optimize the trampoline
as much as possible, which is why (iirc) I had asked for the use of
SHLD here. Considering David's work to split boot and permanent
trampoline I'm find with the minimal 1 byte increase though:

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


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