Subject: [linux-sunxi] Re: [PATCH] crypto: sun4i-ss: prevent deadlock on emulated hardware

From: [email-masked] (Corentin Labbe)
Date: Fri, 15 Jun 2018 11:16:50 +0200

On Fri, Jun 15, 2018 at 11:04:12AM +0200, Maxime Ripard wrote:
> On Fri, Jun 15, 2018 at 10:15:54AM +0200, Corentin Labbe wrote:
> > On Fri, Jun 15, 2018 at 09:57:54AM +0200, Maxime Ripard wrote:
> > > On Thu, Jun 14, 2018 at 09:36:59PM +0200, Corentin Labbe wrote:
> > > > Running a qemu emulated cubieboard with sun4i-ss driver enabled led to a never
> > > > ending boot.
> > > > This is due to sun4i-ss deadlocked and taking all cpu in an infinite loop.
> > > > Since the crypto hardware is not implemented, all registers are read as 0.
> > > > So sun4i-ss will never progress in any operations. (TX_CNT being always 0)
> > > > 
> > > > The first idea is to add a "TX_CNT always zero timeout" but this made cipher/hash loops
> > > > more complex and prevent a case that never happen on real hardware.
> > > > 
> > > > The best way to fix is to check at probe time if we run on a virtual
> > > > machine with hardware emulated but non-implemented and prevent
> > > > sun4i-ss to be loaded in that case.
> > > > Letting sun4i-ss to load is useless anyway since all crypto algorithm will be
> > > > disabled since they will fail crypto selftests.
> > > > 
> > > > Tested-on: qemu-cubieboard
> > > > Tested-on: cubieboard2
> > > > 
> > > > Signed-off-by: Corentin Labbe <[email-masked]>
> > > > ---
> > > >  drivers/crypto/sunxi-ss/sun4i-ss-core.c | 10 ++++++++++
> > > >  1 file changed, 10 insertions(+)
> > > > 
> > > > diff --git a/drivers/crypto/sunxi-ss/sun4i-ss-core.c b/drivers/crypto/sunxi-ss/sun4i-ss-core.c
> > > > index a81d89b3b7d8..a178e80adcf3 100644
> > > > --- a/drivers/crypto/sunxi-ss/sun4i-ss-core.c
> > > > +++ b/drivers/crypto/sunxi-ss/sun4i-ss-core.c
> > > > @@ -341,9 +341,18 @@ static int sun4i_ss_probe(struct platform_device *pdev)
> > > >  	 * I expect to be a sort of Security System Revision number.
> > > >  	 * Since the A80 seems to have an other version of SS
> > > >  	 * this info could be useful
> > > > +	 * Detect virtual machine with non-implemented hardware
> > > > +	 * (qemu-cubieboard) by checking the register value after a write to it.
> > > > +	 * On non-implemented hardware, all registers are read as 0.
> > > > +	 * On real hardware we should have a value > 0.
> > > >  	 */
> > > >  	writel(SS_ENABLED, ss->base + SS_CTL);
> > > >  	v = readl(ss->base + SS_CTL);
> > > > +	if (!v) {
> > > > +		dev_err(&pdev->dev, "Qemu with non-implemented SS detected.\n");
> > > > +		err = -ENODEV;
> > > > +		goto error_rst;
> > > > +	}
> > > 
> > > This is wrong way to tackle the issue. There's multiple reason why
> > > this could happen (for example the device not being clocked, or
> > > maintained in reset). There's nothing specific about qemu here, and
> > > the fundamental issue isn't that the device isn't functional in qemu,
> > > it's that qemu lies about which hardware it can emulate in the DT it
> > > passes to the kernel.
> > > 
> > > There's no way this can scale, alone from the fact that qemu should
> > > patch the DT according to what it can do. Not trying to chase after
> > > each and every device that is broken in qemu.
> > > 
> > > NAK.
> > > 
> > 
> > My fix detect also when the device is badly clocked.
> 
> In which case, the proper fix is to enable the clock, not throw the
> kernel's arm up in the air.
> 

By badly I mean "not clocked" or "with the wrong frequencies".

I could change the clock rate range test to exit (it issue only a warning for now).
But I think this fix detect all cases and still permit someone to play with overclocking/downclocking.

Regards