Re: [PATCH v7 11/17] openrisc: account for 0 starting value in random_get_entropy()

[Stafford Horne]

On Sat, Apr 30, 2022 at 03:29:37AM +0200, Jason A. Donenfeld wrote:
> Hi Stafford,
> 
> On Sat, Apr 30, 2022 at 10:19:03AM +0900, Stafford Horne wrote:
> > Hi Jason,
> > 
> > On Fri, Apr 29, 2022 at 02:16:48AM +0200, Jason A. Donenfeld wrote:
> > > As a sanity check, this series makes sure that during early boot, the
> > > cycle counter isn't returning all zeros. However, OpenRISC's TTCR timer
> > > can be rather slow and starts out as zero during stages of early boot.
> > > We know it works, however. So just always add 1 to random_get_entropy()
> > > so that it doesn't trigger these checks.
> > 
> > Just one nit, you might want to qualify that this is related to simulators/qemu:
> >  * "However, in simulators OpenRISC's TTCR timer can be rather slow..."
> 
> Nice catch, will do.

I was thinking about this, the reason the tick timer is returing 0 is because
the timer is not started.  It's getting initialized right after the random
number generator.

A patch like this helps to startup the timer during intial startup, but I am not
sure its the best thing:

diff --git a/arch/openrisc/kernel/head.S b/arch/openrisc/kernel/head.S
index 15f1b38dfe03..a9b3b5614e13 100644
--- a/arch/openrisc/kernel/head.S
+++ b/arch/openrisc/kernel/head.S
@@ -521,6 +521,9 @@ _start:
        l.ori   r3,r0,0x1
        l.mtspr r0,r3,SPR_SR
 
+       l.movhi r3,hi(SPR_TTMR_CR)
+       l.mtspr r0,r3,SPR_TTMR
+
        CLEAR_GPR(r1)
        CLEAR_GPR(r2)
        CLEAR_GPR(r3)

I was testing this by removing the get_cycles() + 1 patch and I get the
following warning. Starting the tick timer early on in kernel boot helps fix
this issue as well.

But I wonder:
 - Why don't any other architectures have similar issues.
 - Is there any more correct place to do an early timer kick off.

    exec: /home/shorne/work/openrisc/qemu/build/or1k-softmmu/qemu-system-or1k -cpu or1200 -M or1k-sim -kernel /home/shorne/work/linux/vmlinux -initrd /home/shorne/work/linux/initramfs.cpio.gz -serial mon:stdi
    o -nographic -gdb tcp::10001 -m 64

    [    0.000000] OpenRISC Linux -- http://openrisc.io
    [    0.000000] percpu: Embedded 5 pages/cpu s11584 r8192 d21184 u40960
    [    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 8160
    [    0.000000] Kernel command line:
    [    0.000000] Dentry cache hash table entries: 8192 (order: 2, 32768 bytes, linear)
    [    0.000000] Inode-cache hash table entries: 4096 (order: 1, 16384 bytes, linear)
    [    0.000000] Sorting __ex_table...
    [    0.000000] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
    [    0.000000] mem_init_done ...........................................
    [    0.000000] Memory: 57424K/65536K available (4595K kernel code, 157K rwdata, 976K rodata, 195K init, 89K bss, 8112K reserved, 0K cma-reserved)
    [    0.000000] rcu: Hierarchical RCU implementation.
    [    0.000000] rcu:     RCU restricting CPUs from NR_CPUS=2 to nr_cpu_ids=1.
    [    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 10 jiffies.
    [    0.000000] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=1
    [    0.000000] NR_IRQS: 32, nr_irqs: 32, preallocated irqs: 0

(Warning for get_cycles returning 0)

    [    0.000000] ------------[ cut here ]------------
    [    0.000000] WARNING: CPU: 0 PID: 0 at drivers/char/random.c:1016 rand_initialize+0x15c/0x18c
    [    0.000000] Missing cycle counter and fallback timer; RNG entropy collection will consequently suffer.
    [    0.000000] Modules linked in:
    [    0.000000] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.18.0-rc4-17398-gc576c8ccdc30-dirty #724
    [    0.000000] Call trace:
    [    0.000000] [<(ptrval)>] dump_stack_lvl+0x78/0xa0
    [    0.000000] [<(ptrval)>] dump_stack+0x1c/0x2c
    [    0.000000] [<(ptrval)>] __warn+0x100/0x13c
    [    0.000000] [<(ptrval)>] ? rand_initialize+0x15c/0x18c
    [    0.000000] [<(ptrval)>] warn_slowpath_fmt+0x84/0x9c
    [    0.000000] [<(ptrval)>] rand_initialize+0x15c/0x18c
    [    0.000000] [<(ptrval)>] ? start_kernel+0x5dc/0x7c4
    [    0.000000] [<(ptrval)>] ? start_kernel+0x0/0x7c4
    [    0.000000] ---[ end trace 0000000000000000 ]---

(Start of OpenRISC tick timer)

    [    0.000000] clocksource: openrisc_timer: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 95563022313 ns
    [    0.000000] 40.00 BogoMIPS (lpj=200000)
    [    0.000000] pid_max: default: 32768 minimum: 301
    [    0.000000] random: get_random_bytes called from net_ns_init+0x94/0x3f8 with crng_init=0
    [    0.000000] Mount-cache hash table entries: 2048 (order: 0, 8192 bytes, linear)
    [    0.000000] Mountpoint-cache hash table entries: 2048 (order: 0, 8192 bytes, linear)
    [    0.010000] rcu: Hierarchical SRCU implementation.