Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on movdir64b CPU instruction

[Dave Jiang]

On 11/20/19 5:22 PM, Thomas Gleixner wrote:
> On Wed, 20 Nov 2019, Luck, Tony wrote:
> > On Wed, Nov 20, 2019 at 10:53:39PM +0100, Borislav Petkov wrote:
> > > On Wed, Nov 20, 2019 at 02:23:49PM -0700, Dave Jiang wrote:
> > > > +static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
> > > > +				    size_t count)
> > >
> > > An iosubmit function which returns void and doesn't tell its callers
> > > whether it succeeded or not? That looks non-optimal to say the least.
> >
> > That's the underlying functionality of the MOVDIR64B instruction. A
> > posted write so no way to know if it succeeded. When using dedicated
> > queues the caller must keep count of how many operations are in flight
> > and not send more than the depth of the queue.
> >
> > > Why isn't there a fallback function which to call when the CPU doesn't
> > > support movdir64b?
> >
> > This particular driver has no option for fallback. Descriptors can
> > only be submitted with MOVDIR64B (to dedicated queues ... in later
> > patch series support for shared queues will be added, but those require
> > ENQCMD or ENQCMDS to submit).
> >
> > The driver bails out at the beginning of the probe routine if the
> > necessary instructions are not supported:
> >
> > +       /*
> > +        * If the CPU does not support write512, there's no point in
> > +        * enumerating the device. We can not utilize it.
> > +        */
> > +       if (!cpu_has_write512())
> > +               return -ENXIO;
> >
> > Though we should always get past that as this PCI device ID shouldn't
> > every appear on a system that doesn't have the support. Device is on
> > the die, not a plug-in card.
>
> Then the condition in the iosubmit function is just prone to silently paper
> over any bug in a driver:
>
> > +       if (!cpu_has_write512())
> > +               return;
>
> This should at least issue a WARN_ON_ONCE()

Thanks! I'll be adding the WARN_ON_ONCE() for the cap check. Also with 
the alignment check Borislav mentioned, I'll add a WARN_ON() for failures.


> Thanks,
>
> 	tglx