From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Magenheimer Subject: RE: [RFC] Physical hot-add cpus and TSC Date: Wed, 26 May 2010 09:44:15 -0700 (PDT) Message-ID: <5738eada-ede6-4a58-af6b-a7f965aa3ecf@default> References: <3c6e0679-a0b8-42ae-9e86-36e7fa260e73@default C82300BD.15EF3%keir.fraser@eu.citrix.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 , "Xen-Devel (xen-devel@lists.xensource.com)" List-Id: xen-devel@lists.xenproject.org > From: Keir Fraser [mailto:keir.fraser@eu.citrix.com] > Subject: Re: [Xen-devel] [RFC] Physical hot-add cpus and TSC >=20 > On 26/05/2010 16:19, "Dan Magenheimer" > wrote: >=20 > > Much of the TSC-based time infrastructure in Xen, > > especially as exposed to guests, is rather sensitive > > to sudden dramatic differences in TSC values between > > physical processors. Hot-add of physical CPUs will > > introduce a huge difference. >=20 > True at the moment, but can we not just whack the TSC of the newly > added CPU on the head when it is brought online, to match the > boot CPU? Possibly... but the code for whacking the TSC of a CPU after C3-state results in a TSC value that is poorly-aligned with other running TSCs. If there is a better way for "whacking" that results in a nearly-perfectly-aligned TSC (that would pass a "tsc warp test"), that is an option. > I think that would suffice for systems with 'reliable tsc' > which are the only ones we don't emulate tsc by default? Yes, I'm particularly concerned with hot-add-physical-cpu on any latest generation QPI/HT boxes where Invariant TSC is set.