Re: [PATCH] scsi: sd: Contribute to randomness when running rotational device

[Maciej Åenczykowski]

On Sun, Sep 9, 2018 at 4:52 AM, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> On Thu, Sep 06, 2018 at 01:37:19PM -0700, Xuewei Zhang wrote:
>> Currently a scsi device won't contribute to kernel randomness when it
>> uses blk-mq. Since we commonly use scsi on rotational device with
>> blk-mq, it make sense to keep contributing to kernel randomness in these
>> cases. This is especially important for virtual machines.
>>
>> commit b5b6e8c8d3b4 ("scsi: virtio_scsi: fix IO hang caused by automatic
>> irq vector affinity") made all virtio-scsi device to use blk-mq, which
>> does not contribute to randomness today. So for a virtual machine only
>> having virtio-scsi disk (which is common), it will simple stop getting
>> randomness from its disks in today's implementation.
>>
>> With this patch, if the above VM has rotational virtio-scsi device, then
>> it can still benefit from the entropy generated from the disk.
>>
>> Reported-by: Xuewei Zhang <xueweiz@xxxxxxxxxx>
>> Signed-off-by: Xuewei Zhang <xueweiz@xxxxxxxxxx>
>> ---
>>  drivers/scsi/sd.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
>> index b79b366a94f7..5e4f10d28065 100644
>> --- a/drivers/scsi/sd.c
>> +++ b/drivers/scsi/sd.c
>> @@ -2959,6 +2959,9 @@ static void sd_read_block_characteristics(struct scsi_disk *sdkp)
>>       if (rot == 1) {
>>               blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
>>               blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, q);
>> +     } else {
>> +             blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
>> +             blk_queue_flag_set(QUEUE_FLAG_ADD_RANDOM, q);
>>       }
>>
>>       if (sdkp->device->type == TYPE_ZBC) {
>> --
>> 2.19.0.rc2.392.g5ba43deb5a-goog
>>
>
> Look reasonable, especially the disk randomness is added by SCSI itself.
>
> Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
>
> Thanks,
> Ming

Also, see: https://bugzilla.redhat.com/show_bug.cgi?id=1572944
where we're having randomness starvation problems on FC28
running 4.18.5 due to lack of virtio-rng device in VM.
(VM boot takes 9+ hours or 2 haswell VMs)

I'd kindly request we get this not only into 4.19 but also stable trees.
(along with Ted's other randomization fixes)

Reviewed-by: Maciej Åenczykowski <maze@xxxxxxxxxx>