[Bug 238794] LAPIC register test failure with OpenBSD 6.5-snapshot guest

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238794

            Bug ID: 238794
           Summary: LAPIC register test failure with OpenBSD 6.5-snapshot
                    guest
           Product: Base System
           Version: 11.3-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bhyve
          Assignee: virtualization at FreeBSD.org
          Reporter: jason at tubnor.net

OpenBSD has just imported LLVM 8.0 into -current and the way the compiler
builds code to test the LAPIC register has changed (though still valid
according to Intel documentation).

guenther@ from the OpenBSD project provided some insights based on information
provided.  Below is an extract from the email thread:

> Below is output from bhyve.log:
> 
> rdmsr to register 0xc80 on vcpu 0
> Failed to emulate instruction [0xf7 0x04 0x25 0x00 0xd3 0xd1 0x81 0x00 0x10
> 0x00 0x00 0x74 0x08 0xf3 0x90] at 0xffffffff817648f0

According to objdump -d, that's:
 f7 04 25 00 d3 d1 81    testl  $0x1000,0xffffffff81d1d300
 00 10 00 00
 74 08                   je     <forward some>
 f3 90                   pause

That's testing the LAPIC ICRLO, a memory-mapped register.  Previously, the 
compiler generated code like this:

 8b 0c 25 00 63 cf 81    mov    0xffffffff81cf6300,%ecx
 f7 c1 00 10 00 00       test   $0x1000,%ecx
 74 09                   je     <forward some>
 f3 90                   pause  

where it loaded the LAPIC register into %ecx and then tested that value; 
now it combines them and does a direct test.  Congrats, that's legal 
according to Intel (reportedly, Windows will use SSE(!) instructions to 
read LAPIC registers), so this seems like a bug in Bhyve.

The full thread can be found here:

https://marc.info/?l=openbsd-bugs&m=156142900518812&w=2

-- 
You are receiving this mail because:
You are the assignee for the bug.