linux-kernel - Re: [PATCH 1/2] kvm: Fix x86 emulator writeback

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Message-ID: <46766D57.8040206@qumranet.com>
Date:	Mon, 18 Jun 2007 14:32:39 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Luca Tettamanti <kronos.it@...il.com>
CC:	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kvm: Fix x86 emulator writeback

Avi Kivity wrote:
> Luca Tettamanti wrote:
>> When the old value and new one are the same the emulator skips the
>> write; this is undesiderable when the destination is a MMIO area and the
>> write shall be performed regardless of the previous value. This
>> optimization breaks e.g. a Linux guest APIC compiled without
>> X86_GOOD_APIC.
>>
>> Remove the check and always perform the writeback stage in the
>> emulation.
>>
>>   
>
> Unfortunately, this kills Windows XP (first run with a guest crash, 
> second with a host oops), so I reverted it.  I'd guess some operation  
> which doesn't need writeback ends up in the modified code.  
> Previously, the check caused it to skip writeback, but now it writes 
> back random memory, causing a crash.
>

There are comments around like

>     /* Disable writeback. */
>     dst.orig_val = dst.val;

Best option is probably to add an explicit disable_writeback flag and 
set it there.

-- 
error compiling committee.c: too many arguments to function

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