Re: [PATCH v2 2/4] target/ppc: init 'lpcr' in kvmppc_enable_cap_large_decr()

[Daniel Henrique Barboza <danielhb413@gmail.com>]

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.

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? */


CONFIG_VALGRIND_H needs 'valgrind-devel´ installed.

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.


Thanks,


Daniel



> 
>> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>> Signed-off-by: Daniel Henrique Barboza <danielhb413@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? */
>