Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Received-SPF: None (esa4.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  andrew.cooper3@citrix.com) identity=pra;
  client-ip=162.221.158.21; receiver=esa4.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="andrew.cooper3@citrix.com";
  x-conformance=sidf_compatible
Received-SPF: Pass (esa4.hc3370-68.iphmx.com: domain of
  Andrew.Cooper3@citrix.com designates 162.221.158.21 as
  permitted sender) identity=mailfrom;
  client-ip=162.221.158.21; receiver=esa4.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="Andrew.Cooper3@citrix.com";
  x-conformance=sidf_compatible; x-record-type="v=spf1";
  x-record-text="v=spf1 ip4:209.167.231.154 ip4:178.63.86.133
  ip4:195.66.111.40/30 ip4:85.115.9.32/28 ip4:199.102.83.4
  ip4:192.28.146.160 ip4:192.28.146.107 ip4:216.52.6.88
  ip4:216.52.6.188 ip4:162.221.158.21 ip4:162.221.156.83 ~all"
Received-SPF: None (esa4.hc3370-68.iphmx.com: no sender
  authenticity information available from domain of
  postmaster@mail.citrix.com) identity=helo;
  client-ip=162.221.158.21; receiver=esa4.hc3370-68.iphmx.com;
  envelope-from="Andrew.Cooper3@citrix.com";
  x-sender="postmaster@mail.citrix.com";
  x-conformance=sidf_compatible
IronPort-SDR: yhwOshEczxToX4P2UvFZh0XJbI9iVXUVvjRFuasfC5vnGQoWxx2yG04LjZCV+0ZvDKgsz9yTrP
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 bn8=
Subject: Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h
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>
References: <776cb5c2d33e7fd0d2893904724c0e52b394f24a.1565817448.git.thomas.lendacky@amd.com>
 <a24a2c7d-cfab-a049-37e8-7260a9063a7c@citrix.com>
 <20190815210547.GL15313@zn.tnic>
 <312b307b-19cc-84f8-97e6-07dbdf07dd12@citrix.com>
 <20190817084410.GA15364@zn.tnic>
From:   Andrew Cooper <andrew.cooper3@citrix.com>
Openpgp: preference=signencrypt
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Message-ID: <919c80f1-53a5-44d2-d785-88890e449b38@citrix.com>
Date:   Sat, 17 Aug 2019 12:43:52 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
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In-Reply-To: <20190817084410.GA15364@zn.tnic>
Content-Type: text/plain; charset="utf-8"
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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