Add a global synchronization point for pvclock
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From: Jeremy Fitzhardinge on 23 Apr 2010 17:40 On 04/23/2010 02:31 PM, Zachary Amsden wrote: > Does lfence / mfence actually serialize? I thought there was some > great confusion about that not being the case on all AMD processors, > and possibly not at all on Intel. > > A trap, however is a great way to serialize. > > I think, there is no serializing instruction which can be used from > userspace which does not trap, at least, I don't know one off the top > of my head. rsm is not technically privileged... but not quite usable from usermode ;) J -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Zachary Amsden on 23 Apr 2010 17:40 On 04/22/2010 11:34 PM, Avi Kivity wrote: > On 04/23/2010 04:44 AM, Zachary Amsden wrote: >> Or apply this patch. >> time-warp.patch >> >> >> 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. > Does lfence / mfence actually serialize? I thought there was some great confusion about that not being the case on all AMD processors, and possibly not at all on Intel. A trap, however is a great way to serialize. I think, there is no serializing instruction which can be used from userspace which does not trap, at least, I don't know one off the top of my head. Zach -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Zachary Amsden on 23 Apr 2010 17:50 On 04/23/2010 11:35 AM, Jeremy Fitzhardinge wrote: > On 04/23/2010 02:31 PM, Zachary Amsden wrote: > >> Does lfence / mfence actually serialize? I thought there was some >> great confusion about that not being the case on all AMD processors, >> and possibly not at all on Intel. >> >> A trap, however is a great way to serialize. >> >> I think, there is no serializing instruction which can be used from >> userspace which does not trap, at least, I don't know one off the top >> of my head. >> > rsm is not technically privileged... but not quite usable from usermode ;) > rsm under hardware virtualization makes my head hurt -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Avi Kivity on 24 Apr 2010 05:30 On 04/24/2010 12:31 AM, Zachary Amsden wrote: > On 04/22/2010 11:34 PM, Avi Kivity wrote: >> On 04/23/2010 04:44 AM, Zachary Amsden wrote: >>> Or apply this patch. >>> time-warp.patch >>> >>> >>> 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. >> > > Does lfence / mfence actually serialize? I thought there was some > great confusion about that not being the case on all AMD processors, > and possibly not at all on Intel. They don't. > > A trap, however is a great way to serialize. > > I think, there is no serializing instruction which can be used from > userspace which does not trap, at least, I don't know one off the top > of my head. iret. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Avi Kivity on 24 Apr 2010 05:40
On 04/24/2010 12:41 AM, Zachary Amsden wrote: >> rsm is not technically privileged... but not quite usable from >> usermode ;) > > > rsm under hardware virtualization makes my head hurt Either one independently is sufficient for me. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |