linux-kernel - Re: [PATCH] x86_64: use HPET in 64-bit mode to avoid a harware 32-bit wraparound bug

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Message-ID: <alpine.DEB.2.20.1712281243110.1688@nanos>
Date:   Thu, 28 Dec 2017 12:49:22 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jiri Bohac <jbohac@...e.cz>
cc:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64: use HPET in 64-bit mode to avoid a harware 32-bit
 wraparound bug

On Fri, 8 Dec 2017, Jiri Bohac wrote:

> This patch changes the HPET clockevent code to use the Timer 0 in 64-bit
> mode (if compiled for x86_64; doing this for 32-bit kernels would bring
> a performance penalty as a single read would require three 32-bit HPET
> accesses).

We tried that before and 64bit mode did not work on all tested platforms.

Aside of that your patch is broken because it does not check whether HPET
actually supports 64bit mode at all. The COUNT_SIZE_CAP bit is there for a
reason.

Thanks,

	tglx