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

tglx

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