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

[Bart Van Assche]

On Thu, 2018-09-06 at 13:37 -0700, Xuewei Zhang wrote:
+AD4 Currently a scsi device won't contribute to kernel randomness when it
+AD4 uses blk-mq. Since we commonly use scsi on rotational device with
+AD4 blk-mq, it make sense to keep contributing to kernel randomness in these
+AD4 cases. This is especially important for virtual machines.
+AD4 
+AD4 commit b5b6e8c8d3b4 (+ACI-scsi: virtio+AF8-scsi: fix IO hang caused by automatic
+AD4 irq vector affinity+ACI) made all virtio-scsi device to use blk-mq, which
+AD4 does not contribute to randomness today. So for a virtual machine only
+AD4 having virtio-scsi disk (which is common), it will simple stop getting
+AD4 randomness from its disks in today's implementation.
+AD4 
+AD4 With this patch, if the above VM has rotational virtio-scsi device, then
+AD4 it can still benefit from the entropy generated from the disk.
+AD4 
+AD4 Reported-by: Xuewei Zhang +ADw-xueweiz+AEA-google.com+AD4
+AD4 Signed-off-by: Xuewei Zhang +ADw-xueweiz+AEA-google.com+AD4
+AD4 ---
+AD4  drivers/scsi/sd.c +AHw 3 +-+-+-
+AD4  1 file changed, 3 insertions(+-)
+AD4 
+AD4 diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
+AD4 index b79b366a94f7..5e4f10d28065 100644
+AD4 --- a/drivers/scsi/sd.c
+AD4 +-+-+- b/drivers/scsi/sd.c
+AD4 +AEAAQA -2959,6 +-2959,9 +AEAAQA static void sd+AF8-read+AF8-block+AF8-characteristics(struct
+AD4 scsi+AF8-disk +ACo-sdkp)
+AD4  	if (rot +AD0APQ 1) +AHs
+AD4  		blk+AF8-queue+AF8-flag+AF8-set(QUEUE+AF8-FLAG+AF8-NONROT, q)+ADs
+AD4  		blk+AF8-queue+AF8-flag+AF8-clear(QUEUE+AF8-FLAG+AF8-ADD+AF8-RANDOM, q)+ADs
+AD4 +-	+AH0 else +AHs
+AD4 +-		blk+AF8-queue+AF8-flag+AF8-clear(QUEUE+AF8-FLAG+AF8-NONROT, q)+ADs
+AD4 +-		blk+AF8-queue+AF8-flag+AF8-set(QUEUE+AF8-FLAG+AF8-ADD+AF8-RANDOM, q)+ADs
+AD4  	+AH0
+AD4  
+AD4  	if (sdkp-+AD4-device-+AD4-type +AD0APQ TYPE+AF8-ZBC) +AHs

Although this patch looks fine to me, seeing this patch makes me wonder
whether the default should be changed (QUEUE+AF8-FLAG+AF8-MQ+AF8-DEFAULT) instead of
modifying the sd driver. Can anyone remind me why QUEUE+AF8-FLAG+AF8-MQ+AF8-DEFAULT does
not include QUEUE+AF8-FLAG+AF8-ADD+AF8-RANDOM?

Thanks,

Bart.