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

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Message-ID: <alpine.DEB.2.21.1911210120410.29534@nanos.tec.linutronix.de>
Date:   Thu, 21 Nov 2019 01:22:41 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Luck, Tony" <tony.luck@...el.com>
cc:     Borislav Petkov <bp@...en8.de>, Dave Jiang <dave.jiang@...el.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        vkoul@...nel.org, dan.j.williams@...el.com, jing.lin@...el.com,
        ashok.raj@...el.com, sanjay.k.kumar@...el.com, megha.dey@...el.com,
        jacob.jun.pan@...el.com, yi.l.liu@...el.com, axboe@...nel.dk,
        akpm@...ux-foundation.org, mingo@...hat.com, fenghua.yu@...el.com,
        hpa@...or.com
Subject: Re: [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