linux-kernel - Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

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Date:   Sat, 17 Aug 2019 12:43:52 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, Chen Yu <yu.c.chen@...el.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

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