Re: [Qemu-devel] [RFC/PATCH] qemu: Use valgrind annotations to mark kvm guest memory as defined

[Avi Kivity]

On 08/21/2012 12:03 PM, Christian Borntraeger wrote:
> On 10/08/12 15:11, Christian Borntraeger wrote:
>> valgrind with kvm produces a big amount of false positives regarding
>> "Conditional jump or move depends on uninitialised value(s)". This
>> happens because the guest memory is allocated with qemu_vmalloc which
>> boils down posix_memalign etc. This function is (correctly) considered
>> by valgrind as returning undefined memory.
>> 
>> Since valgrind is based on jitting code, it will not be able to see
>> changes made by the guest to guest memory if this is done by KVM_RUN,
>> thus keeping most of the guest memory undefined.
>> 
>> Now lots of places in qemu will then use guest memory to change behaviour.
>> To avoid the flood of these messages, lets declare the whole guest
>> memory as defined. This will reduce the noise and allows us to see real
>> problems.
>> 
>> In the future we might want to make this conditional, since there
>> is actually something that we can use those false positives for:
>> These messages will point to code that depends on guest memory, so
>> we can use these backtraces to actually make an audit that is focussed
>> only at those code places. For normal development we dont want to
>> see those messages, though.
> 
> 
> Any opinion about this patch?

Missed it, sorry.  Now applied to uq/master, thanks.


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