linux-kernel - Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD

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Date:   Tue, 7 Jul 2020 16:04:50 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Babu Moger <babu.moger@....com>
Cc:     fenghua.yu@...el.com, stable@...r.kernel.org, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, tglx@...utronix.de,
        reinette.chatre@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD

On Wed, Jul 01, 2020 at 05:13:45PM -0500, Babu Moger wrote:
> [ Upstream commit 2c18bd525c47f882f033b0a813ecd09c93e1ecdf ]
> 
> Memory bandwidth is calculated reading the monitoring counter
> at two intervals and calculating the delta. It is the software’s
> responsibility to read the count often enough to avoid having
> the count roll over _twice_ between reads.
> 
> The current code hardcodes the bandwidth monitoring counter's width
> to 24 bits for AMD. This is due to default base counter width which
> is 24. Currently, AMD does not implement the CPUID 0xF.[ECX=1]:EAX
> to adjust the counter width. But, the AMD hardware supports much
> wider bandwidth counter with the default width of 44 bits.
> 
> Kernel reads these monitoring counters every 1 second and adjusts the
> counter value for overflow. With 24 bits and scale value of 64 for AMD,
> it can only measure up to 1GB/s without overflowing. For the rates
> above 1GB/s this will fail to measure the bandwidth.
> 
> Fix the issue setting the default width to 44 bits by adjusting the
> offset.
> 
> AMD future products will implement CPUID 0xF.[ECX=1]:EAX.
> 
>  [ bp: Let the line stick out and drop {}-brackets around a single
>    statement. ]
> 
> Fixes: 4d05bf71f157 ("x86/resctrl: Introduce AMD QOS feature")
> Signed-off-by: Babu Moger <babu.moger@....com>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> Link: https://lkml.kernel.org/r/159129975546.62538.5656031125604254041.stgit@naples-babu.amd.com
> ---
> 
> Note:
>  This commit is already queued for 5.7 stable kernel.
>  Backporting it t 5.6 stable and older kernels now.

Now queued up to 5.4.y, thanks.

greg k-h