RE: [Xen-devel] Problem with BIOS timer interrupts

["Huang2, Wei" <Wei.Huang2@xxxxxxx>]

Gary,

Which CPU family you are using? 0xF? There is an errata which seems to
be related. See page 50. 

http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/
33610.pdf


-Wei
                

-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Gary Grebus
Sent: Tuesday, November 18, 2008 12:50 PM
To: xen-devel
Subject: [Xen-devel] Problem with BIOS timer interrupts

Hi,

While changing our Xen 3.2.x based HVM BIOS ROM to use gPXE instead of
etherboot, I ran into an interesting behavior.  The gPXE code, which
runs in real mode, contains the following sequence:

wait_for_tick:
        pushl   %eax
        pushw   %fs
        movw    $0x40, %ax
        movw    %ax, %fs
        movl    %fs:(0x6c), %eax
1:      pushf
        sti
        hlt
        popf
        cmpl    %fs:(0x6c), %eax
        je      1b
        popw    %fs
        popl    %eax
        ret

It uses this to timeout waiting for a key press.  The expected interrupt
is from the BIOS timer implemented in rombios.  But in fact, the loop
hangs.  However, if I insert a nop instruction between the sti and hlt,
then things work as expected.

Is there something wrong with this sequence?  This happens on AMD, so
it's not a quirk of the real mode emulations on Intel. 

I notice that in the gPXE code currently in xen-unstable, the path that
uses this code is patched out.

        /gary

-- 
Gary Grebus
Virtual Iron Software, Inc.



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