Message-ID: <8a7775912f31394944b43db12adc78efd84b1fad.camel@HansenPartnership.com>
Subject: Re: [regression] Bug 216989 - since 6.1 systems with AMD Ryzen
 stutter when fTPM is enabled
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Linux regressions mailing list <regressions@lists.linux.dev>,
        Peter Huewe <peterhuewe@gmx.de>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Jason Gunthorpe <jgg@ziepe.ca>
Cc:     linux-integrity@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, reach622@mailcuk.com,
        1138267643@qq.com, "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Thu, 02 Feb 2023 07:57:37 -0500
In-Reply-To: <3a196414-68d8-29c9-24cc-2b8cb4c9d358@leemhuis.info>
References: <3a196414-68d8-29c9-24cc-2b8cb4c9d358@leemhuis.info>
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On Thu, 2023-02-02 at 11:28 +0100, Linux kernel regression tracking
(Thorsten Leemhuis) wrote:
[...]
> So it's a firmware problem, but apparently one that Linux only
> triggers since 6.1.
> 
> Jason, could the hwrng changes have anything to do with this?
> 
> A bisection really would be helpful, but I guess that is not easy as
> the problem apparently only shows up after some time...

the problem description says the fTPM causes system stutter when it
writes to NVRAM.  Since an fTPM is a proprietary implementation, we
don't know what it does.  The ms TPM implementation definitely doesn't
trigger NV writes on rng requests, but it is plausible this fTPM does
... particularly if they have a time based input to the DRNG.  Even if
this speculation is true, there's not much we can do about it, since
it's a firmware bug and AMD should have delivered the BIOS update that
fixes it.

The way to test this would be to set the config option

CONFIG_HW_RANDOM_TPM=n

and see if the stutter goes away.  I suppose if someone could quantify
the bad bioses, we could warn, but that's about it.

James