linux-kernel - RE: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and x86

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Date:   Thu, 30 Jun 2022 09:43:36 +0000
From:   "tarumizu.kohei@...itsu.com" <tarumizu.kohei@...itsu.com>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        Linus Walleij <linus.walleij@...aro.org>
CC:     Greg KH <gregkh@...uxfoundation.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
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        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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        <linux-arm-kernel@...ts.infradead.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        Paolo Valente <paolo.valente@...more.it>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and
 x86

Hi Dave,

> They run their fortran app.  Change the MSRs.  Run it again.  See if it
> simulated the nuclear weapon blast any faster or slower.  Rinse.  Repeat.
> 
> One thing that is missing from the changelog and cover letter here: On x86,
> there's a 'wrmsr(1)' tool.  That took pokes at Model Specific Registers (MSRs)
> via the /dev/cpu/X/msr interface.  That interface is a very, very thinly-veiled
> wrapper around the WRMSR (WRite MSR) instruction.
> 
> In other words, on x86, our current interface allows userspace programs to
> arbitrarily poke at our most sensitive hardware configuration registers.  One of
> the most common reasons users have reported doing this (we have
> pr_warn()ings about it) is controlling the prefetch hardware.
> 
> This interface would take a good chunk of the x86 wrmsr(1) audience and
> convert them over to a less dangerous interface.  That's a win on x86.
> We don't even *remotely* have line-of-sight for a generic solution for the kernel
> to figure out a single "best" value for these registers.

Thank you for mentioning wrmsr tool.

This is one of the reason why I want to add the sysfs interface. I
will add the description that this interface can be used instead of
wrmsr tool (or MSR driver) for hardware prefetch control usage to the
cover letter.

I read below that we should not accesse any MSR directly from
userspace without restriction.
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/about/