From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Borislav Petkov <bp@alien8.de>
Cc: "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>,
Thomas Gleixner <tglx@linutronix.de>,
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: Sat, 17 Aug 2019 12:43:52 +0100 [thread overview]
Message-ID: <919c80f1-53a5-44d2-d785-88890e449b38@citrix.com> (raw)
In-Reply-To: <20190817084410.GA15364@zn.tnic>
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.
~Andrew
next prev parent reply other threads:[~2019-08-17 11:43 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 [this message]
2019-08-18 16:32 ` Thomas Gleixner
2019-08-16 15:19 ` Andy Lutomirski
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