Re: Oopse in kvm

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

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Avi Kivity wrote:
> David Brown wrote:
>   
>> I was wondering of anyone could help out on this, just found an oopse
>> when trying to start rhel4 on i386 box. I was wondering if anyone else
>> has seen this with kvm-18 and if I should submit a bug for it.
>>
>> [10937.520000] BUG: unable to handle kernel NULL pointer dereference
>> [10937.521000] EIP: [<f9004eec>] mmu_page_remove_parent_pte+0xc/0xf0
>> [kvm] SS:ESP 0068:c918fbbc
>> [10937.521000]
>>
>>   
>>     
>
>
> I've seen a few reports like this, but never managed to get one of my own.
>
> Is this reproducible?  If so, a recipe would be _very_  valuable.
>
> At what stage in the boot process does it crash?
>
>   

Okay, I got a testcase through a private mail which reproduced this on 
guest boot.  The attached patch fixes it.

Note that the problem is only with nonpae guests, so if your guest is 
pae, you're probably seeing something different.

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


[-- Attachment #2: nonpae-oops-fix.patch --]
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diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index b872f09..c814394 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -1190,6 +1190,7 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
 			 * and zap two pdes instead of one.
 			 */
 			if (level == PT32_ROOT_LEVEL) {
+				page_offset &= ~7; /* kill rounding error */
 				page_offset <<= 1;
 				npte = 2;
 			}

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