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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Feng Tang <feng.tang@intel.com>, Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        andi.kleen@intel.com, dave.hansen@intel.com, len.brown@intel.com
Subject: Re: [RFC 1/2] x86/tsc: add a timer to make sure tsc_adjust is always checked
In-Reply-To: <20210410094752.GB21691@zn.tnic>
References: <1617092747-15769-1-git-send-email-feng.tang@intel.com> <87y2dq32xc.ffs@nanos.tec.linutronix.de> <20210410094752.GB21691@zn.tnic>
Date:   Sat, 10 Apr 2021 20:43:27 +0200
Message-ID: <87fszy2d68.ffs@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Sat, Apr 10 2021 at 11:47, Borislav Petkov wrote:

> On Sat, Apr 10, 2021 at 11:27:11AM +0200, Thomas Gleixner wrote:
>> On Tue, Mar 30 2021 at 16:25, Feng Tang wrote:
>> > Normally the tsc_sync will be checked every time system enters idle state,
>> > but there is still caveat that a system won't enter idle, either because
>> > it's too busy or configured purposely to not enter idle. Setup a periodic
>> > timer to make sure the check is always on.
>> 
>> Bah. I really hate the fact that we don't have a knob to disable writes
>> to the TSC/TSC_ADJUST msrs. That would spare this business alltogether.
>
> We have the MSR filtering and I'd *love* to add those MSRs to a
> permanent ban list of MSRs which will never ever be written to from
> luserspace.

That's good, but what I really want is a knob which prevents BIOS/SMM
from writing to it. The only reason why BIOS ever needs to write is for
physical hotplug and perhaps for 4+ socket machines on boot. After that
every write is a bug.

Thanks,

        tglx