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[Bug 1699867] Re: x86_64 qemu crashes at far call into long-mode

From: Launchpad Bug Tracker
Subject: [Bug 1699867] Re: x86_64 qemu crashes at far call into long-mode
Date: Sat, 16 Jan 2021 04:17:50 -0000

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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  x86_64 qemu crashes at far call into long-mode

Status in QEMU:

Bug description:
  I am experimenting with this OS https://github.com/phil-opp/blog_os
  and spotted a weird behavior with qemu.

  I am looking at code that does transition from 32bit mode to 64bit
  mode. Right now it does 'jmp $SELECTOR,64bitfunction'.

  This code works fine with qemu/kvm/vmware.

  To transition from 32 to 64 bit code it is possible to use 'call'
  operation. So I am trying to replace that code with 'call
  $SELECTOR,64bitfunction'. It works fine with kvm and wmware. But it
  fails with Qemu emulation. See the interrup debug log attached. qemu
  crashes at 10302c (far call mnemonic).

    103016:       e8 18 00 00 00          callq  103033 <set_up_page_tables>
    10301b:       e8 5c 00 00 00          callq  10307c <enable_paging>
    103020:       e8 ec 00 00 00          callq  103111 <set_up_SSE>
    103025:       0f 01 15 28 00 10 00    lgdt   0x100028(%rip)        # 203054 
    10302c:       9a                      (bad)  
    10302d:       40 31 10                rex xor %edx,(%rax)
    103030:       00 08                   add    %cl,(%rax)

  As the code works at hardware I expect it to work with qemu. Thus current 
qemu behavior looks like a bug.

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