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

[Paolo Bonzini <pbonzini@redhat.com>]

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

> 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)
>