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From: "Sebastian Herbszt" <herbszt@gmx.de>
References: <488DC8B2.1070009@redhat.com>
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Date: Thu, 31 Jul 2008 21:45:41 +0200
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Subject: [Qemu-devel] Re: Re: [PATCH 0/3]: Add UUID command-line option
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To: qemu-devel@nongnu.org, Jamie Lokier <jamie@shareable.org>
Cc: Gleb Natapov <gleb@qumranet.com>, Chris Lalancette <clalance@redhat.com>

Jamie Lokier wrote:
>> >void uuid_probe(void)
>> >{
>> >#ifdef BX_QEMU
>> >    uint32_t eax, ebx, ecx, edx;
>> >
>> >    // check if backdoor port exists
>> >    asm volatile ("outl %%eax, %%dx"
>> >        : "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx)
>> >        : "a" (0x564d5868), "c" (0xa), "d" (0x5658));
>> >    if (ebx == 0x564d5868) {
> 
> Has a bug.
> 
> EBX is not initialised prior to the ASM, and could contain the same
> value as EAX.
> 
> If the I/O doesn't do anything (like on a real PC), it could match the
> condition which says its a backdoor.
> 
> The ASM should initialise EBX to something else:
> 
>   asm volatile ("outl %%eax, %%dx"
>       : "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx)
>       : "a" (0x564d5868), "b" (0), "c" (0xa), "d" (0x5658));

Fixed in cvs (rombios32.c r1.29) now. Thanks for spotting it.

- Sebastian