From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock Date: Fri, 23 Apr 2010 22:25:50 +0300 Message-ID: <4BD1F43E.2080404@redhat.com> References: <1271675100.1674.818.camel@laptop> <4BCC3A3E.9070909@redhat.com> <20100419142158.GD14158@mothafucka.localdomain> <4BCC69D5.3050209@redhat.com> <1271688411.1488.248.camel@laptop> <4BCC8246.9040202@goop.org> <4BCD748E.7080007@redhat.com> <4BCDF12C.1020702@goop.org> <4BCDF85C.3000004@redhat.com> <4BCE0399.2010708@goop.org> <20100422131113.GA3364@mothafucka.localdomain> <4BD0FB92.4060301@redhat.com> <4BD169B5.4000409@redhat.com> <4BD1F37A.6040801@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Zachary Amsden , Glauber Costa , Peter Zijlstra , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Marcelo Tosatti To: Jeremy Fitzhardinge Return-path: In-Reply-To: <4BD1F37A.6040801@goop.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 04/23/2010 10:22 PM, Jeremy Fitzhardinge wrote: > On 04/23/2010 02:34 AM, Avi Kivity wrote: > >>> diff -rup a/time-warp-test.c b/time-warp-test.c >>> --- a/time-warp-test.c 2010-04-15 16:30:13.955981607 -1000 >>> +++ b/time-warp-test.c 2010-04-15 16:35:37.777982377 -1000 >>> @@ -91,7 +91,7 @@ static inline unsigned long long __rdtsc >>> { >>> DECLARE_ARGS(val, low, high); >>> >>> - asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high)); >>> + asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high) :: >>> "ebx", "ecx"); >>> >>> >>> >> >> Plus, replace cpuid by lfence/mfence. cpuid will trap. >> > I presume the clobbers are needed for cpuid; if you use [lm]fence then > they shouldn't be needed, right? > > Right. But I'm not sure lfence/mfence are sufficient - from what I understand rdtsc can pass right through them. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.