Re: arch/xen is a bad idea

From: Ian Pratt (Ian.Pratt_at_cl.cam.ac.uk)
Date: 12/16/04

  • Next message: Jurriaan: "Re: 2.6 flavours" 
    To: Andi Kleen <ak@suse.de>
Date:	Thu, 16 Dec 2004 20:37:11 +0000

    > > There are so many problems in snooping and decoding instructions it
    > > isn't funny. Aside from the mmap pci buffer half way through instruction
    >
    > Hmm? From what I remember reading the code of the Xen hypervisor
    > they are already emulating a lot of instructions (e.g. take a look
    > at xen/arch/x86/x86_32/seg_fixup.c) Emulating some more doesn't
    > seem like a big issue to me.

    There's actually no emulation in Xen at all. seg_fixup is the
    closest we come, which is a tiny decoder that is able to work out
    the effective address of an instruction. It's only through grim
    necessity we need this in order to be able to make NPTL thread
    local storage accesses work (-ve segment offsets). We'd much
    prefer that glibc was slightly tweaked such that we didn't have
    to do this (it would be a lot faster too).

    Anyhow, I really don't think emulation is the issue. The handful
    of cases that you pointed out that could relatively easily be
    emulated constitute a tiny fraction of the arch xen patch. Even
    so, you have to be a bit careful from a performance point of
    view: loading debug registers can occur quite frequently, and its
    much quicker to be able to load them as a batch rather than
    taking individual faults.

    Ian
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

  • Next message: Jurriaan: "Re: 2.6 flavours"

    Relevant Pages

    • Re: VAX/VMS Consulting in Europe
      ... CMPCs, MOVTUC, MATCHC, etc. were. ... extendec accuracy with 29 H-floating and octaword instructions. ... which instructions to implement or emulation on a per-instruction basis ...
      (comp.os.vms)
    • Re: linux
      ... The VAX needs an executable ... Maybe these architecture from bygone ddidn't use a bios. ... Your emulation code needs to cover the processor instructions, ...
      (comp.os.linux)
    • Re: Sun Studio option to generate quad-precision fp instructions on SPARC ?
      ... that triggers FP emulation on any multiprocessor system. ... execution in-order with the surrounding instructions (Ok, ... generic sparcv9 code should be sufficient in this case. ... Not at the price of nearly uselessly slow execution. ...
      (comp.unix.solaris)
    • Re: your mail
      ... > instructions are emulated on the older CPUs, ... There was a patch around a while back that did 486 emulation on 386 ... I think it even made into the Mandrake kernel. ... send the line "unsubscribe linux-kernel" in ...
      (Linux-Kernel)
    • Does PB 5.0 work with VS 2005 Beta 2 and WM 2005?
      ... my existing PB 5.0 fails to run images under emulation ... (this appears to be by design, ... instructions to uninstall PB entirely). ...
      (microsoft.public.windowsce.platbuilder)