linux-kernel - Re: [PATCH] x86/vdso: Remove direct HPET access through the vDSO

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Date:	Sat, 9 Apr 2016 15:47:47 -0700 (PDT)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...nel.org>
cc:	Borislav Petkov <bp@...en8.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Waiman Long <waiman.long@....com>
Subject: Re: [PATCH] x86/vdso: Remove direct HPET access through the vDSO

On Thu, 7 Apr 2016, Andy Lutomirski wrote:

> Allowing user code to map the HPET is problematic.  HPET
> implementations are notoriously buggy, and there are probably many
> machines on which even MMIO reads from bogus HPET addresses are
> problematic.
> 
> We have a report that the Dell Precision M2800 with:
> 
> ACPI: HPET 0x00000000C8FE6238 000038 (v01 DELL   CBX3  01072009 AMI. 00000005)
> 
> is either so slow when accessing the HPET or actually hangs in some
> regard, causing soft lockups to be reported if users do unexpected
> things to the HPET.
> 
> The vclock HPET code has also always been a questionable speedup.
> Accessing an HPET is exceedingly slow (on the order of several
> microseconds), so the added overhead in requiring a syscall to read
> the HPET is a small fraction of the total code of accessing it.
> 
> To avoid future problems, let's just delete the code entirely.
> 
> In the long run, this could actually be a speedup.  Waiman Long as a
> patch to optimize the case where multiple CPUs contend for the HPET,
> but that won't help unless all the accesses are mediated by the
> kernel.
> 
> Cc: Waiman Long <waiman.long@....com>
> Reported-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>