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

Christian Borntraeger
2019-07-31T12:43:50Z

On 31.07.19 14:28, Christian Borntraeger wrote:
>
>
> On 31.07.19 14:04, Andrey Shinkevich wrote:
>> On 31/07/2019 10:24, Christian Borntraeger wrote:
>>>
>>>
>>> On 30.07.19 21:20, Paolo Bonzini wrote:
>>>> On 30/07/19 18:01, 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>
>>>>
>>>> Christian, is this the right fix? It's not expensive so it wouldn't be
>>>> an issue, just checking if there's any better alternative.
>>>
>>> I think all of these variants are valid with pros and cons
>>> 1. teach valgrind about this:
>>> Add to coregrind/m_syswrap/syswrap-linux.c (and the relevant header files)
>>> knowledge about which parts are actually touched.
>>> 2. use designated initializers
>>> 3. use memset
>>> 3. use a valgrind callback VG_USERREQ__MAKE_MEM_DEFINED to tell that this memory is defined
>>>
>>
>> Thank you all very much for taking part in the discussion.
>> Also, one may use the Valgrind technology to suppress the unwanted
>> reports by adding the Valgrind specific format file valgrind.supp to the
>> QEMU project. The file content is extendable for future needs.
>> All the cases we like to suppress will be recounted in that file.
>> A case looks like the stack fragments. For instance, from QEMU block:
>>
>> {
>> hw/block/hd-geometry.c
>> Memcheck:Cond
>> fun:guess_disk_lchs
>> fun:hd_geometry_guess
>> fun:blkconf_geometry
>> ...
>> fun:device_set_realized
>> fun:property_set_bool
>> fun:object_property_set
>> fun:object_property_set_qobject
>> fun:object_property_set_bool
>> }
>>
>> The number of suppressed cases are reported by the Valgrind with every
>> run: "ERROR SUMMARY: 5 errors from 3 contexts (suppressed: 0 from 0)"
>>
>> Andrey
>
> Yes, indeed that would be another variant. How performance critical are
> the fixed locations? That might have an impact on what is the best solution.
> From a cleanliness approach doing 1 (adding the ioctl definition to valgrind)
> is certainly the most beautiful way. I did that in the past, look for example at
>
> https://sourceware.org/git/?p=valgrind.git;a=commitdiff;h=c2baee9b7bf043702c130de0771a4df439fcf403
> or
> https://sourceware.org/git/?p=valgrind.git;a=commitdiff;h=00a31dd3d1e7101b331c2c83fca6c666ba35d910
>
> for examples.
>
>
>>
>>>>
>>>> Paolo
>>>>
>>>>> ---
>>>>> 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));
>>>>> 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));

This is memsetting 4k?
Yet another variant would be to use the RUNNING_ON_VALGRIND macro from
valgrind/valgrind.h to only memset for valgrind. But just using MAKE_MEM_DEFINED
from memcheck.h is simpler.