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From: Thomas Gleixner <tglx@linutronix.de>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Borislav Petkov <bp@alien8.de>,
	"Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Pavel Machek <pavel@ucw.cz>, Chen Yu <yu.c.chen@intel.com>,
	Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h
Date: Sun, 18 Aug 2019 18:32:29 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1908181831340.1923@nanos.tec.linutronix.de> (raw)
In-Reply-To: <919c80f1-53a5-44d2-d785-88890e449b38@citrix.com>

On Sat, 17 Aug 2019, Andrew Cooper wrote:
> On 17/08/2019 09:44, Borislav Petkov wrote:
> > On Thu, Aug 15, 2019 at 10:25:24PM +0100, Andrew Cooper wrote:
> >> I'm afraid that a number of hypervisors do write-discard, given the
> >> propensity of OSes (certainly traditionally) to go poking at bits like
> >> this without wrmsr_safe().
> >>
> >> You either need to read the MSR back and observe that the bit has really
> >> changed, or in this case as Thomas suggests, look at CPUID again (which
> >> will likely be the faster option for the non-virtualised case).
> > One thing I didn't think of when we talked about this: this happens only
> > after you resume the hypervisor.
> 
> :) It hadn't escaped my notice, hence the intervention on this thread.
> 
> > And the words "resume the hypervisor" already means an improbable use case.
> 
> Qubes and OpenXT are two laptop+hypervisor oriented distros where
> suspend/resume is a big deal, and these will have to follow AMD's
> recommendation here.
> 
> However, for servers which don't do S3/S4, we can reason about safely
> leaving RDRAND enabled, irrespective of guest configuration.

Let the administrator reason about it. Default is off for sanity sake.

Thanks,

	tglx

  reply	other threads:[~2019-08-18 16:32 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-14 21:17 [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h Lendacky, Thomas
2019-08-14 23:24 ` Non-random RDRAND " Pavel Machek
2019-08-14 23:38   ` Pavel Machek
2019-08-15 13:01   ` Lendacky, Thomas
2019-08-15 15:12   ` Theodore Y. Ts'o
2019-08-16  9:07     ` Pavel Machek
2019-08-16 14:42     ` Neil Horman
2019-08-15  7:21 ` Borislav Petkov
2019-08-15 13:47   ` Lendacky, Thomas
2019-08-15 15:34     ` Borislav Petkov
2019-08-15 20:14       ` Thomas Gleixner
2019-08-15 20:59 ` Andrew Cooper
2019-08-15 21:04   ` Thomas Gleixner
2019-08-15 21:05   ` Borislav Petkov
2019-08-15 21:25     ` Andrew Cooper
2019-08-17  8:44       ` Borislav Petkov
2019-08-17 11:43         ` Andrew Cooper
2019-08-18 16:32           ` Thomas Gleixner [this message]
2019-08-16 15:19 ` Andy Lutomirski

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