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

Andrey Shinkevich
2019-07-31T14:11:10Z

On 31/07/2019 15:43, Christian Borntraeger wrote:
>
>
> 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.
>

So, on this assumption, the code would look like

#ifdef CONFIG_VALGRIND_H
#include <valgrind/memcheck.h>
#endif

#ifdef CONFIG_VALGRIND_H
VALGRIND_MAKE_MEM_DEFINED(&msr_data, sizeof(msr_data));
#endif

etc.

Andrey
--
With the best regards,
Andrey Shinkevich