Re: [Qemu-devel] [PATCH 3/3] i386/kvm: initialize struct at full before ioctl call

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 7/30/19 7:05 PM, Christian Borntraeger wrote:
> On 30.07.19 18:44, Philippe Mathieu-Daudé wrote:
>> On 7/30/19 6:01 PM, Andrey Shinkevich wrote:
>>> Not the whole structure is initialized before passing it to the KVM.
>>> Reduce the number of Valgrind reports.
>>>
>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>> ---
>>>  target/i386/kvm.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>>> index dbbb137..ed57e31 100644
>>> --- a/target/i386/kvm.c
>>> +++ b/target/i386/kvm.c
>>> @@ -190,6 +190,7 @@ static int kvm_get_tsc(CPUState *cs)
>>>          return 0;
>>>      }
>>>  
>>> +    memset(&msr_data, 0, sizeof(msr_data));
>>
>> I wonder the overhead of this one...
> 
> Cant we use designated initializers like in
> 
> commit bdfc8480c50a53d91aa9a513d23a84de0d5fbc86
> Author:     Christian Borntraeger <borntraeger@de.ibm.com>
> AuthorDate: Thu Oct 30 09:23:41 2014 +0100
> Commit:     Paolo Bonzini <pbonzini@redhat.com>
> CommitDate: Mon Dec 15 12:21:01 2014 +0100
> 
>     valgrind/i386: avoid false positives on KVM_SET_XCRS ioctl
> 
> and others?

Is the compiler smart enough to figure out it doesn't need to zeroes in
case env->tsc_valid is true and the function returns?

> 
> This should minimize the impact. 
>>
>>>      msr_data.info.nmsrs = 1;
>>>      msr_data.entries[0].index = MSR_IA32_TSC;
>>>      env->tsc_valid = !runstate_is_running();
>>> @@ -1706,6 +1707,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>>  
>>>      if (has_xsave) {
>>>          env->xsave_buf = qemu_memalign(4096, sizeof(struct kvm_xsave));
>>> +        memset(env->xsave_buf, 0, sizeof(struct kvm_xsave));
>>
>> OK
>>
>>>      }
>>>  
>>>      max_nested_state_len = kvm_max_nested_state_length();
>>> @@ -3477,6 +3479,7 @@ static int kvm_put_debugregs(X86CPU *cpu)
>>>          return 0;
>>>      }
>>>  
>>> +    memset(&dbgregs, 0, sizeof(dbgregs));
>>
>> OK
>>
>>>      for (i = 0; i < 4; i++) {
>>>          dbgregs.db[i] = env->dr[i];
>>>      }
>>
>> We could remove 'dbgregs.flags = 0;'
>>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>
>