On Thu, Mar 31, 2022 at 02:17:42PM -0300, Daniel Henrique Barboza wrote: > > > On 3/30/22 22:25, David Gibson wrote: > > On Wed, Mar 30, 2022 at 09:17:15PM -0300, Daniel Henrique Barboza wrote: > > > 'lpcr' is used as an input of kvm_get_one_reg(). Valgrind doesn't > > > understand that and it returns warnings as such for this function: > > > > > > ==55240== Thread 1: > > > ==55240== Conditional jump or move depends on uninitialised value(s) > > > ==55240== at 0xB011E4: kvmppc_enable_cap_large_decr (kvm.c:2546) > > > ==55240== by 0x92F28F: cap_large_decr_cpu_apply (spapr_caps.c:523) > > > ==55240== by 0x930C37: spapr_caps_cpu_apply (spapr_caps.c:921) > > > ==55240== by 0x955D3B: spapr_reset_vcpu (spapr_cpu_core.c:73) > > > ==55240== by 0x95612B: spapr_cpu_core_reset (spapr_cpu_core.c:209) > > > ==55240== by 0x95619B: spapr_cpu_core_reset_handler > > > (spapr_cpu_core.c:218) > > > ==55240== by 0xD3605F: qemu_devices_reset (reset.c:69) > > > ==55240== by 0x92112B: spapr_machine_reset (spapr.c:1641) > > > ==55240== by 0x4FBD63: qemu_system_reset (runstate.c:444) > > > ==55240== by 0x62812B: qdev_machine_creation_done (machine.c:1247) > > > ==55240== by 0x5064C3: qemu_machine_creation_done (vl.c:2725) > > > ==55240== by 0x5065DF: qmp_x_exit_preconfig (vl.c:2748) > > > ==55240== Uninitialised value was created by a stack allocation > > > ==55240== at 0xB01158: kvmppc_enable_cap_large_decr (kvm.c:2540) > > > > > > Init 'lpcr' to avoid this warning. > > > > Hmm... this is seeming a bit like whack-a-mole. Could we instead use > > one of the valgrind hinting mechanisms to inform it that > > kvm_get_one_reg() writes the variable at *target? > > I didn't find a way of doing that looking in the memcheck helpers > (https://valgrind.org/docs/manual/mc-manual.html section 4.7). That would be a > good way of solving this warning because we would put stuff inside a specific > function X and all callers of X would be covered by it. > > What I did find instead is a memcheck macro called VALGRIND_MAKE_MEM_DEFINED > that > tells Valgrind that the var was initialized.
I think that's the one I was thinking of. > This patch would then be something as follows: > > > diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c > index dc93b99189..b0e22fa283 100644 > --- a/target/ppc/kvm.c > +++ b/target/ppc/kvm.c > @@ -56,6 +56,10 @@ > #define DEBUG_RETURN_GUEST 0 > #define DEBUG_RETURN_GDB 1 > +#ifdef CONFIG_VALGRIND_H > +#include <valgrind/memcheck.h> > +#endif > + > const KVMCapabilityInfo kvm_arch_required_capabilities[] = { > KVM_CAP_LAST_INFO > }; > @@ -2539,6 +2543,10 @@ int kvmppc_enable_cap_large_decr(PowerPCCPU *cpu, int > enable) > CPUState *cs = CPU(cpu); > uint64_t lpcr; > +#ifdef CONFIG_VALGRIND_H > + VALGRIND_MAKE_MEM_DEFINED(lpcr, sizeof(uint64_t)); > +#endif > + > kvm_get_one_reg(cs, KVM_REG_PPC_LPCR_64, &lpcr); > /* Do we need to modify the LPCR? */ The macro call should only go after the get_one_reg, of course. > CONFIG_VALGRIND_H needs 'valgrind-devel´ installed. Right.. better would probably be to make a wrapper macro defined as a no-op in the !CONFIG_VALGRIND_H case, so you don't need the ifdefs at the point you use it. > > I agree that this "Valgrind is complaining about variable initialization" is > a whack-a-mole > situation that will keep happening in the future if we keep adding this same > code pattern > (passing as reference an uninitialized var). For now, given that we have only > 4 instances > to fix it in ppc code (as far as I'm aware of), and we don't have a better > way of telling > Valgrind that we know what we're doing, I think we're better of > initializing these vars. Hmm... still feels like it would be better to put the MAKE_MEM_DEFINED inside kvm_get_one_reg(). I think the difficulty with that is that it handles both 32-bit and 64-bit registers and I'm not sure if there's an easy way to work out exactly how many bits *have* been initialized. > > > Thanks, > > > Daniel > > > > > > > > Reviewed-by: Philippe Mathieu-Daudé <f4...@amsat.org> > > > Signed-off-by: Daniel Henrique Barboza <danielhb...@gmail.com> > > > --- > > > target/ppc/kvm.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c > > > index 858866ecd4..42814e1b97 100644 > > > --- a/target/ppc/kvm.c > > > +++ b/target/ppc/kvm.c > > > @@ -2538,7 +2538,7 @@ int kvmppc_get_cap_large_decr(void) > > > int kvmppc_enable_cap_large_decr(PowerPCCPU *cpu, int enable) > > > { > > > CPUState *cs = CPU(cpu); > > > - uint64_t lpcr; > > > + uint64_t lpcr = 0; > > > kvm_get_one_reg(cs, KVM_REG_PPC_LPCR_64, &lpcr); > > > /* Do we need to modify the LPCR? */ > > > -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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