From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [RFC PATCH 0/2] x86: Fix missing core serialization on migration Date: Tue, 14 Nov 2017 17:08:42 +0100 Message-ID: <20171114160842.GH3857@worktop> References: <20171110211249.10742-1-mathieu.desnoyers@efficios.com> <885227610.13045.1510351034488.JavaMail.zimbra@efficios.com> <617343212.13932.1510592207202.JavaMail.zimbra@efficios.com> <4d47fbb8-8f99-19d3-a9cf-66841aeffac3@scylladb.com> <4431530.14831.1510672632887.JavaMail.zimbra@efficios.com> <20171114160541.GC3165@worktop.lehotels.local> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20171114160541.GC3165-IIpfhp3q70x9+YH6RuovlLjjLBE8jN/0@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Mathieu Desnoyers Cc: Avi Kivity , Linus Torvalds , Andy Lutomirski , linux-kernel , linux-api , "Paul E. McKenney" , Boqun Feng , Andrew Hunter , maged michael , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrea Parri , "Russell King, ARM Linux" , Greg Hackmann List-Id: linux-api@vger.kernel.org On Tue, Nov 14, 2017 at 05:05:41PM +0100, Peter Zijlstra wrote: > On Tue, Nov 14, 2017 at 03:17:12PM +0000, Mathieu Desnoyers wrote: > > I've tried to create a small single-threaded self-modifying loop in > > user-space to trigger a trace cache or speculative execution quirk, > > but I have not succeeded yet. I suspect that I would need to know > > more about the internals of the processor architecture to create the > > right stalls that would allow speculative execution to move further > > ahead, and trigger an incoherent execution flow. Ideas on how to > > trigger this would be welcome. > > I thought the whole problem was per definition multi-threaded. > > Single-threaded stuff can't get out of sync with itself; you'll always > observe your own stores. And even if you could, you can always execute a local serializing instruction like CPUID to force things. > And ISTR the JIT scenario being something like the JIT overwriting > previously executed but supposedly no longer used code. And in this > scenario you'd want to guarantee all CPUs observe the new code before > jumping into it. > > The current approach is using mprotect(), except that on a number of > platforms the TLB invalidate from that is not guaranteed to be strong > enough to sync for code changes. > > On x86 the mprotect() should work just fine, since we broadcast IPIs for > the TLB invalidate and the IRET from those will get the things synced up > again (if nothing else; very likely we'll have done a MOV-CR3 which will > of course also have sufficient syncness on it). > > But PowerPC, s390, ARM et al that do TLB invalidates without interrupts > and don't guarantee their TLB invalidate sync against execution units > are left broken by this scheme. >