From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method Date: Fri, 10 Feb 2017 19:01:52 +0100 (CET) Message-ID: References: <20170209141052.18694-1-vkuznets@redhat.com> <20170209141052.18694-3-vkuznets@redhat.com> <87d1eqqlkv.fsf@vitty.brq.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Stephen Hemminger Cc: Haiyang Zhang , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , Andy Lutomirski , Ingo Molnar , "H. Peter Anvin" , "devel@linuxdriverproject.org" , "virtualization@lists.linux-foundation.org" List-Id: virtualization@lists.linuxfoundation.org On Fri, 10 Feb 2017, Stephen Hemminger wrote: > Since sequence count algorithm is done by hypervisor, better to not reuse seqcount. > Still concerned that the code is racy. That's a different question and can only be answered by the hypervisor folks. Dunno, whether they have barrier requirements. The seqcount stuff relies on: do { seq = READ_ONCE(s->sequence); smp_rmb(); sample_data(); smp_rmb(); } while (s->sequence != seq); which pairs with the writer side: s->sequence++; smp_wmb(); update_data(); smp_wmb(); s->sequence++; That's important if the stuff happens cross CPU. If the update happens on the same CPU then this is a different story and as there are VMexits involved they might provide the required ordering already. But I can't tell as I have no idea how that host side thing is done. Thanks, tglx