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From: Thomas Gleixner <tglx@linutronix.de>
To: bp@alien8.de, dwmw@amazon.co.uk, gregkh@linux-foundation.org,
	thomas.lendacky@amd.com, pjt@google.com, mingo@kernel.org,
	linux-kernel@vger.kernel.org, hpa@zytor.com,
	tim.c.chen@linux.intel.com, torvalds@linux-foundation.org,
	peterz@infradead.org, dave.hansen@intel.com
Cc: linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC
Date: Mon, 8 Jan 2018 11:08:55 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.20.1801081105120.1735@nanos> (raw)
In-Reply-To: <tip-0bf17c102177d5da9363bf8b1e4704b9996d5079@git.kernel.org>

On Sat, 6 Jan 2018, tip-bot for Tom Lendacky wrote:

> Commit-ID:  0bf17c102177d5da9363bf8b1e4704b9996d5079
> Gitweb:     https://git.kernel.org/tip/0bf17c102177d5da9363bf8b1e4704b9996d5079
> Author:     Tom Lendacky <thomas.lendacky@amd.com>
> AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600
> Committer:  Thomas Gleixner <tglx@linutronix.de>
> CommitDate: Sat, 6 Jan 2018 21:57:40 +0100
> 
> x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC
> 
> With LFENCE now a serializing instruction, set the LFENCE_RDTSC
> feature since the LFENCE instruction has less overhead than the
> MFENCE instruction.

Second thoughts on that. As pointed out by someone in one of the insane
long threads:

What happens if the kernel runs as a guest and

  - the hypervisor did not set the LFENCE to serializing on the host

  - the hypervisor does not allow writing MSR_AMD64_DE_CFG

That would bring the guest into a pretty bad state or am I missing
something essential here?

I'm dropping these patches until this question is answered.

Thanks,

	tglx

  reply	other threads:[~2018-01-08 10:09 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-05 16:07 [PATCH v1 0/3] x86/cpu/AMD: Make LFENCE a serializing instruction on AMD Tom Lendacky
2018-01-05 16:07 ` [PATCH v1 1/3] x86/cpu/AMD: Make LFENCE a serializing instruction Tom Lendacky
2018-01-05 16:35   ` Brian Gerst
2018-01-05 16:36     ` Tom Lendacky
2018-01-06 21:05   ` [tip:x86/pti] " tip-bot for Tom Lendacky
2018-01-05 16:07 ` [PATCH v1 2/3] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC Tom Lendacky
2018-01-06 21:06   ` [tip:x86/pti] " tip-bot for Tom Lendacky
2018-01-08 10:08     ` Thomas Gleixner [this message]
2018-01-08 10:23       ` Woodhouse, David
2018-01-08 10:25         ` Thomas Gleixner
2018-01-08 10:40       ` Andrew Cooper
2018-01-08 11:10         ` Thomas Gleixner
2018-01-08 14:47           ` Tom Lendacky
2018-01-08 14:54             ` Andrew Cooper
2018-01-08 16:48               ` Dr. David Alan Gilbert
2018-01-08 17:01                 ` Paolo Bonzini
2018-01-08 17:39                   ` Tom Lendacky
2018-01-08 17:42                     ` Paolo Bonzini
2018-01-17 17:21                   ` Tom Lendacky
2018-01-17 17:53                     ` Paolo Bonzini
2018-01-08 15:02             ` David Woodhouse
2018-01-08 15:15             ` Thomas Gleixner
2018-01-08 17:31               ` Tom Lendacky
2018-01-08 20:57                 ` Thomas Gleixner
2018-01-05 16:08 ` [PATCH v1 3/3] x86/msr: Remove now unused definition of MFENCE_RDTSC feature Tom Lendacky
2018-01-06 21:06   ` [tip:x86/pti] " tip-bot for Tom Lendacky
2018-01-05 16:56 ` [PATCH v1 0/3] x86/cpu/AMD: Make LFENCE a serializing instruction on AMD Borislav Petkov

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