[llvm-mca][docs] clarify how the quality of the perf report is affected by the quality of the scheduling models.

Differential Revision: https://reviews.llvm.org/D63556


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363830 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/llvm-mca.rst

@@ -38,6 +38,17 @@ Or for Intel syntax:
 
   $ clang foo.c -O2 -target x86_64-unknown-unknown -mllvm -x86-asm-syntax=intel -S -o - | llvm-mca -mcpu=btver2
 
+Scheduling models are not just used to compute instruction latencies and
+throughput, but also to understand what processor resources are available
+and how to simulate them.
+
+By design, the quality of the analysis conducted by :program:`llvm-mca` is
+inevitably affected by the quality of the scheduling models in LLVM.
+
+If you see that the performance report is not accurate for a processor,
+please `file a bug <https://bugs.llvm.org/enter_bug.cgi?product=libraries>`_
+against the appropriate backend.
+
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