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

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Date:   Wed, 20 Nov 2019 17:32:51 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, "Luck, Tony" <tony.luck@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, dmaengine@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vinod Koul <vkoul@...nel.org>, Jing Lin <jing.lin@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, Jens Axboe <axboe@...nel.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on
 movdir64b CPU instruction

On Wed, Nov 20, 2019 at 4:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, 20 Nov 2019, Dan Williams wrote:
> > On Wed, Nov 20, 2019 at 3:27 PM Borislav Petkov <bp@...en8.de> wrote:
> > >
> > > On Wed, Nov 20, 2019 at 03:19:23PM -0800, Luck, Tony wrote:
> > > > That's the underlying functionality of the MOVDIR64B instruction. A
> > > > posted write so no way to know if it succeeded.
> > >
> > > So how do you know whether any of the writes went through?
> >
> > It's identical to the writel() mmio-write to start a SATA command
> > transfer. The higher level device driver protocol validates that the
> > command went through, ultimately with a timeout. There's no return
> > value for iosubmit_cmds512() for the same reason there's no return
> > value for the other iowrite primitives.
>
> With the difference that other iowrite primitive have no dependencies on
> cpu feature bits and cannot fail on the software level.

True, but that would be a driver coding mistake flagged by the
WARN_ON_ONCE, and the failure is static. The driver must check for
static_cpu_has(X86_FEATURE_MOVDIR64B) once at init, but it need not
check again on every command submission.