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

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Message-ID: <20190817084410.GA15364@zn.tnic>
Date:   Sat, 17 Aug 2019 10:44:10 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
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 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. And the words "resume the hypervisor"
already means an improbable use case. Yeah, yeah, one can close the
laptop lid of her/his F15h or F16h machine while guests are running and
when the HV resumes, those guests won't get randomness but I can't seem
to find it in myself to say, uuh, that's an important use case...

-- 
Regards/Gruss,
    Boris.

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