From 5131afcd078876180216f03ec99ab60f665f5a93 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Ciro=20Santilli=20=E5=85=AD=E5=9B=9B=E4=BA=8B=E4=BB=B6=20?= =?UTF-8?q?=E6=B3=95=E8=BD=AE=E5=8A=9F?= Date: Sun, 27 Oct 2019 00:00:00 +0000 Subject: [PATCH] cb6ea177715a43aeaa8314b4e7109535417d2315 --- index.html | 155 +++++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 126 insertions(+), 29 deletions(-) diff --git a/index.html b/index.html index d43f6ca..2e74972 100644 --- a/index.html +++ b/index.html @@ -658,6 +658,7 @@ body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-b
  • 9. KVM
  • 10. User mode simulation @@ -1204,10 +1205,15 @@ body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-b
  • 19.19.5. gem5 stats internals
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  • 19.19.6. gem5 build system +
  • 19.19.6. gem5 code generation +
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  • 19.19.7. gem5 build system +
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    KVM is Linux kernel interface that greatly speeds up execution of virtual machines.

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    You can make QEMU or gem5 by passing enabling KVM with:

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    You can make QEMU or gem5 by passing enabling KVM with:

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    but it was broken in gem5 with pending patches: https://www.mail-archive.com/gem5-users@gem5.org/msg15046.html It fails immediately on:

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    panic: KVM: Failed to enter virtualized mode (hw reason: 0x80000021)
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    KVM works by running userland instructions natively directly on the real hardware instead of running a software simulation of those instructions.

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    TODO: do the right thing and cross compile QEMU and gem5. gem5’s Python parts might be a pain. QEMU should be easy: https://stackoverflow.com/questions/26514252/cross-compile-qemu-for-arm

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    9.2. gem5 KVM

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    While gem5 does have KVM, as of 2019 its support has not been very good, because debugging it is harder and people haven’t focused intensively on it.

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    X86 was broken with pending patches: https://www.mail-archive.com/gem5-users@gem5.org/msg15046.html It failed immediately on:

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    panic: KVM: Failed to enter virtualized mode (hw reason: 0x80000021)
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    Bibliography:

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    The only hairy thing in QEMU is the binary code generation.

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    gem5 however has tended towards intensive code generation in order to support all its different hardware types:

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    gem5 however has tended towards horrendous intensive code generation in order to support all its different hardware types

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    KVM CPUs are an alternative way of fast forwarding boot when they work.

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    19.17.2. gem5 MinorCPU

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    19.19.6. gem5 build system

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    19.19.6. gem5 code generation

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    gem5 uses a ton of code generation, which makes the project horrendous:

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      lots of magic happen on top of pybind11, which is already magic, to more automatically glue the C++ and Python worlds: gem5 Python C++ interaction

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      .isa code which describes most of the instructions

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      Ruby for memory systems

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    To find the definition of generated code, do a:

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    grep -I -r build/ 'code of interest'
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    where:

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    The code generation exists partly to support insanely generic cross ISA instructions mapping to one compute model, where it might be reasonable.

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    But it has been widely overused to insanity. It likely also exists partly because when the project started in 2003 C++ compilers weren’t that good, so you couldn’t rely on features like templates that much.

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    19.19.6.1. gem5 polymorphic ISA includes
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    19.19.6.1. gem5 THE_ISA
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    Generated code at: build/<ISA>/config/the_isa.hh which contains amongst other lines:

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    #define X86_ISA 8
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+enum class Arch {
+  X86ISA = X86_ISA
+};
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+#define THE_ISA X86_ISA
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    Generation code: src/SConscript at def makeTheISA.

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    Tested on gem5 211869ea950f3cc3116655f06b1d46d3fa39fb3a.

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    Bibliography: https://www.mail-archive.com/gem5-users@gem5.org/msg16989.html

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    19.19.7. gem5 build system

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    19.19.7.1. gem5 polymorphic ISA includes

    E.g. src/cpu/decode_cache.hh includes:

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    19.19.6.2. Why are all C++ symlinked into the gem5 build dir?
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    19.19.7.2. Why are all C++ symlinked into the gem5 build dir?

    Some scons madness.

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    It was not possible to disable the symlinks automatically for the entire project when I last asked: https://stackoverflow.com/questions/53656787/how-to-set-disable-duplicate-0-for-all-scons-build-variants-without-repeating-th

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    The horrendous downsides of this are:

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      when debugging the emulator, it shows you directories inside the build directory rather than in the source tree

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      it is harder to separate which files are generated and which are in-tree when grepping for code generated definitions

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