linux-kernel - Re: [PATCH] x86,cpu: add X86_FEATURE

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Message-ID: <628c91f76feaf6e29522c8fef5d9351ba6104183.camel@surriel.com>
Date: Wed, 25 Sep 2024 12:46:39 -0400
From: Rik van Riel <riel@...riel.com>
To: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>, Dave Hansen
 <dave.hansen@...ux.intel.com>,  x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>, linux-kernel@...r.kernel.org,  kernel-team@...a.com
Subject: Re: [PATCH] x86,cpu: add X86_FEATURE_INVLPGB flag

On Wed, 2024-09-25 at 08:45 +0100, Borislav Petkov wrote:
> On September 24, 2024 11:00:13 PM GMT+01:00, Rik van Riel
> <riel@...riel.com> wrote:
> > Add the definition for the X86_FEATURE_INVLPGB CPUID flag.
> > 
> > Tested by booting a kernel with this change on an AMD Milan system,
> > and making sure the "invlpgb" flag shows up in /proc/cpuinfo
> 
> Why?
> 

I would like to avoid TLB flush IPIs for the top N (maybe 8 or 16?)
TLB flushing processes in the system.  We can figure out what these
processes are by keeping a score of TLB flushes * number of CPUs over
some time period.

That seems like something AMD CPUs might be able to do, without
running out of PCIDs even on larger systems.

The TLB flush IPIs are taking a considerable amount of compute
power across a large fleet, and is something I would like to
optimize.

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