Commit 62fe8331 authored Aug 10, 2012 by Christian Borntraeger Committed by Avi Kivity Sep 09, 2012

qemu: Use valgrind annotations to mark kvm guest memory as defined



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.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

parent 0c267217

configure

+2 −1

Original line number	Diff line number	Diff line
		@@ -2963,11 +2963,12 @@ if compile_prog "-Werror" "" ; then
		fi

		########################################
		# check if we have valgrind/valgrind.h
		# check if we have valgrind/valgrind.h and valgrind/memcheck.h

		valgrind_h=no
		cat > $TMPC << EOF
		#include <valgrind/valgrind.h>
		#include <valgrind/memcheck.h>
		int main(void) {
		return 0;
		}

kvm-all.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -39,6 +39,10 @@
		#include <sys/eventfd.h>
		#endif

		#ifdef CONFIG_VALGRIND_H
		#include <valgrind/memcheck.h>
		#endif

		/* KVM uses PAGE_SIZE in its definition of COALESCED_MMIO_MAX */
		#define PAGE_SIZE TARGET_PAGE_SIZE

		@@ -1719,6 +1723,9 @@ void *kvm_vmalloc(ram_addr_t size)

		void kvm_setup_guest_memory(void *start, size_t size)
		{
		#ifdef CONFIG_VALGRIND_H
		VALGRIND_MAKE_MEM_DEFINED(start, size);
		#endif
		if (!kvm_has_sync_mmu()) {
		int ret = qemu_madvise(start, size, QEMU_MADV_DONTFORK);