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Date:   Tue, 6 Sep 2022 23:00:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     "Rafael J . Wysocki" <rafael@kernel.org>,
        Pavel Machek <pavel@ucw.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH] x86/cpu: Avoid writing MSR_IA32_TSX_CTRL when writing it
 is not supported
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References: <20220906201743.436091-1-hdegoede@redhat.com>
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 <6ff23930-325b-4207-12fc-4d8fd5bea1ff@redhat.com>
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On Tue, Sep 06, 2022 at 10:56:47PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 9/6/22 22:43, Peter Zijlstra wrote:
> > On Tue, Sep 06, 2022 at 10:17:43PM +0200, Hans de Goede wrote:
> >> On an Intel Atom N2600 (and presumable other Cedar Trail models)
> >> MSR_IA32_TSX_CTRL can be read, causing saved_msr.valid to be set for it
> >> by msr_build_context().
> >>
> >> This causes restore_processor_state() to try and restore it, but writing
> >> this MSR is not allowed on the Intel Atom N2600 leading to:
> > 
> > FWIW, virt tends to do this same thing a lot. They'll allow reading
> > random MSRs and only fail on write.
> 
> Right. So I guess I should send a v2 with an updated commit
> message mentioning this ?

Nah, just saying this is a somewhat common pattern with MSRs.

The best ones are the one where writing the value read is invalid :/ or
those who also silently eat a 0 write just for giggles. Luckily that
doesn't happen often.