Re: [Qemu-devel] [PATCH v2 7/7] iotests: new file to suppress Valgrind errors

[Andrey Shinkevich]

On 17/06/2019 14:45, Kevin Wolf wrote:
> Am 11.06.2019 um 20:02 hat Andrey Shinkevich geschrieben:
>> The Valgrind tool reports about an uninitialised memory usage when the
>> initialization is actually not needed. For example, the buffer 'buf'
>> instantiated on a stack of the function guess_disk_lchs().
> 
> I would be careful with calling initialisation "not needed". It means
> that the test case may not behave entirely determinstic because the
> uninitialised memory can vary between runs.\

I am going to amend the comment.

Andrey

> 
> In this specific case, I assume that guess_disk_lchs() is called for a
> null block node, for which .bdrv_co_preadv by default returns without
> actually writing to the buffer. Instead of ignoring the valgrind error,
> we could instead pass read-zeroes=on to the null block driver to make
> the test deterministic.

The buffer that the Valgrind complains of is initialized by the 
following function call blk_pread_unthrottled() that reads the first 
BDRV_SECTOR_SIZE bytes form a disk "to guess the disk logical geometry". 
The Valgrind does not recognize that way of initialization. I believe we 
do not need to zero the buffer instantiated on the stack just to make 
the Valgrind silent there.

Andrey

> 
> (Unfortunately, while adding the read-zeroes option, we didn't add it to
> the QAPI schema, so it's not available yet in -blockdev. I'm going to
> send a fix for that, but most of the problematic test cases probably
> don't even use -blockdev.)
> 
> Kevin
> 

-- 
With the best regards,
Andrey Shinkevich