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Date: Mon, 28 Jul 2008 16:48:50 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH 0/3]: Add UUID command-line option
Message-ID: <20080728154849.GA13000@shareable.org>
References: <488DC8B2.1070009@redhat.com> <20080728141515.GJ3196@minantech.com>
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To: qemu-devel@nongnu.org
Cc: Gleb Natapov <gleb@qumranet.com>, Chris Lalancette <clalance@redhat.com>

Anthony Liguori 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));

-- Jamie