From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@qumranet.com>
Subject: Re: [PATCH]: Fix MSR_K7_EVNTSEL{0,3} for SVM
Date: Fri, 25 Apr 2008 10:30:06 +0300
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Cc: kvm-devel <kvm-devel@lists.sourceforge.net>
To: Chris Lalancette <clalance@redhat.com>
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Chris Lalancette wrote:
> Avi Kivity wrote:
>   
>> Now it uses %rsi instead of %esi, and any junk in the upper bits will 
>> cause the ja to be taken.
>>
>> We need to get a reduced testcase to the gcc folks, this is a serious 
>> bug.  Any changes in the code to work around this would be fragile.
>>
>>     
>
> Avi,
>     I've now filed a bug in the upstream gcc database:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36040
>
> And I came up with a reduced test case, available here:
>
> http://people.redhat.com/clalance/rsi-test-case.tar.bz2
>
> If I compile the code in the above and look at the disassembly, it shows the
> problem; however, I can't reproduce the bug by actually running the code.  I
> suspect the %rsi register is always 0 when we start in this userland code, so I
> never run into the bogus ja, but I just thought I'd mention it.
>
>   

Hmm, looking back at the dump:

>     1811:	8d 86 00 00 ff 3f    	lea    0x3fff0000(%rsi),%eax
>     1817:	83 f8 03             	cmp    $0x3,%eax
>     181a:	0f 87 e2 01 00 00    	ja     1a02 <svm_set_msr+0x27f>

So while gcc is using %rsi, it loads the result back into %eax, which 
has the effect of dropping back into 32-bits.  So looks like gcc was 
right here.  Sorry for spreading confusion and apologies to gcc.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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