linux-kernel - Re: soft lockup when passing vvar address to write(2)

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Message-ID: <20160305090432.GB23473@gmail.com>
Date:	Sat, 5 Mar 2016 10:04:33 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: soft lockup when passing vvar address to write(2)


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Fri, 4 Mar 2016, Andy Lutomirski wrote:
> > Thomas, I still think we should consider just deleting the HPET vclock
> > code and accept the syscall overhead on systems that are stuck using
> > HPET.  If fast syscalls are available (which should include every
> > system with HPET, unless there are some 32-bit AMD systems lying
> > around), then the overhead in a syscall is *tiny* compared to the code
> > of the HPET read itself.
> 
> No objection from my side, really.

Seconded. HPET hardware overhead is typically horrifically large in any case, no 
need to memory map it and expose hardware breakages to user-space ...

It's also a (mild) security hole: a well-known HPET address can be abused as a 
statistical trampoline periodically cycling through 'dangerous' instruction 
values.

Thanks,

	Ingo