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

[Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>]

On 31/07/2019 15:32, Paolo Bonzini wrote:
> On 31/07/19 11:05, Christophe de Dinechin wrote:
>>
>> Christian Borntraeger writes:
>>
>>> 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?
>>>
>>> This should minimize the impact.
>>
>> Oh, when you talked about using designated initializers, I thought you
>> were talking about fully initializing the struct, like so:
> 
> Yeah, that would be good too.  For now I'm applying Andrey's series though.
> 
> Paolo
> 

Thank you.
As Philippe wrote, 'dbgregs.flags = 0;' is unnecessary with 'memset(0)'.

Andrey

>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index dbbb13772a..3533870c43 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -180,19 +180,20 @@ static int kvm_get_tsc(CPUState *cs)
>>   {
>>       X86CPU *cpu = X86_CPU(cs);
>>       CPUX86State *env = &cpu->env;
>> -    struct {
>> -        struct kvm_msrs info;
>> -        struct kvm_msr_entry entries[1];
>> -    } msr_data;
>>       int ret;
>>
>>       if (env->tsc_valid) {
>>           return 0;
>>       }
>>
>> -    msr_data.info.nmsrs = 1;
>> -    msr_data.entries[0].index = MSR_IA32_TSC;
>> -    env->tsc_valid = !runstate_is_running();
>> +    struct {
>> +        struct kvm_msrs info;
>> +        struct kvm_msr_entry entries[1];
>> +    } msr_data = {
>> +        .info = { .nmsrs =  1 },
>> +        .entries = { [0] = { .index = MSR_IA32_TSC } }
>> +    };
>> +     env->tsc_valid = !runstate_is_running();
>>
>>       ret = kvm_vcpu_ioctl(CPU(cpu), KVM_GET_MSRS, &msr_data);
>>       if (ret < 0) {
>>
>>
>> This gives the compiler maximum opportunities to flag mistakes like
>> initializing the same thing twice, and make it easier (read no smart
>> optimizations) to initialize in one go. Moving the declaration past the
>> 'if' also addresses Philippe's concern.
>>
>>>>
>>>>>       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>
>>>>
>>
>>
>> --
>> Cheers,
>> Christophe de Dinechin (IRC c3d)
>>
> 

-- 
With the best regards,
Andrey Shinkevich