From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Magenheimer Subject: RE: [PATCH] x86: don't write_tsc() non-zero values on CPUs updating only the lower 32 bits Date: Fri, 15 Apr 2011 07:34:18 -0700 (PDT) Message-ID: References: <7a5514b0-b3d6-478a-9434-b80758a7445d@default C9CDA6D0.16614%keir.xen@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Keir Fraser , Jan Beulich Cc: xen-devel@lists.xensource.com, "winston.l.wang" List-Id: xen-devel@lists.xenproject.org > From: Keir Fraser [mailto:keir.xen@gmail.com] > Sent: Friday, April 15, 2011 12:40 AM > To: Dan Magenheimer; Jan Beulich > Cc: winston.l.wang; xen-devel@lists.xensource.com > Subject: Re: [Xen-devel] [PATCH] x86: don't write_tsc() non-zero values > on CPUs updating only the lower 32 bits >=20 > On 14/04/2011 23:41, "Dan Magenheimer" > wrote: >=20 > >> Yeah, if we want to continue to try avoiding write_tsc() on > >> TSC_RELIABLE > >> then we should assert !TSC_RELIABLE on the write_tsc() path in > >> cstate_tsc_restore(). > > > > Agreed. In fact, maybe it should be asserted in write_tsc? >=20 > We still write_tsc on CPU physical hot-add. Hmmm... IIRC the testing that Intel was doing for hot-add was not for processors that were actually electrically hot-plugged but only for processors that were powered-on at the same time as all other processors but left offline until needed (e.g. for capacity-on-demand). For this situation, writing to tsc is still the wrong approach. I don't think we finished the discussion about electrically hot-plugged processors because they didn't exist... don't know if they do yet either. IIRC I had proposed an unnamed boot parameter that said "this machine may add unsynchronized processors post-boot" and disallow hot-add processors if not specified (or if not specified AND a run-time check of a hot-add processor shows non-synchronization). Dan