From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [RFC PATCH 1/2] Hyper-H reference counter Date: Mon, 20 May 2013 10:56:22 +0200 Message-ID: <5199E536.6070809@redhat.com> References: <1368695621.18400.9.camel@localhost> <20130516092128.GP26453@redhat.com> <1368696535.18400.10.camel@localhost> <5194E2D3.3080300@redhat.com> <1368714390.18400.13.camel@localhost> <5194F0F8.9070205@redhat.com> <1368945475.1859.2.camel@localhost> <5199D952.2020808@redhat.com> <20130520083648.GP4725@redhat.com> <5199E20C.1030004@redhat.com> <20130520084912.GQ4725@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Vadim Rozenfeld , kvm@vger.kernel.org, mtosatti@redhat.com, pl@dlh.net To: Gleb Natapov Return-path: Received: from mx1.redhat.com ([209.132.183.28]:16013 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080Ab3ETI4a (ORCPT ); Mon, 20 May 2013 04:56:30 -0400 In-Reply-To: <20130520084912.GQ4725@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Il 20/05/2013 10:49, Gleb Natapov ha scritto: > On Mon, May 20, 2013 at 10:42:52AM +0200, Paolo Bonzini wrote: >> Il 20/05/2013 10:36, Gleb Natapov ha scritto: >>> On Mon, May 20, 2013 at 10:05:38AM +0200, Paolo Bonzini wrote: >>>> Il 19/05/2013 08:37, Vadim Rozenfeld ha scritto: >>>>> On Thu, 2013-05-16 at 16:45 +0200, Paolo Bonzini wrote: >>>>>> Il 16/05/2013 16:26, Vadim Rozenfeld ha scritto: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Yes, I have this check added in the second patch. >>>>>>>>>>>>>>> >>>>>>>>>>>>> Move it here please. >>>>>>>>>>> OK, will do it. >>>>>>>>> >>>>>>>>> Or better, remove all the handling of HV_X64_MSR_REFERENCE_TSC from this >>>>>>>>> patch, and leave it all to the second. >>>>>>>>> >>>>>>> What for? Could you please elaborate? >>>>> >>>>> To make code reviewable. Add one MSR here, the other in the second patch. >>>>> removing HV_X64_MSR_REFERENCE_TSC will make this particular patch >>>>> completely non-functional. >>>> >>>> Do you mean Windows guest will BSOD or just that they won't use the >>>> reference TSC? If the latter, it's not a problem. >>>> >>> I think it is. If reference counter works without TSC we have a bisect >>> point for the case when something will going wrong with TSC. >> >> Isn't that exactly what might happen with this patch only? Windows will >> not use the TSC because it finds invalid values in the TSC page. > > Yes, it will use reference counter instead. Exactly what we want for a bisect point. > >> If it >> still uses the reference counter, we have the situation you describe. >> >> refcount TSC page >> Y Y <= after patch 2 >> Y N <= after patch 1 >> N Y <= impossible >> N N <= removing TSC page from this patch? >> >> Of course if the guest BSODs, it's not possible to split the patches >> that way. Perhaps in that case it's simply better to do a single patch. >> > I am not sure what you are trying to say. Your option list above shows > that there is a value to split patches like they are split now. Hmm, we're talking past each other. :) I put the "?" because that's what Vadim implied ("it would make this particular patch non-functional"), but I don't see why it should be like this. To me, the obvious way of getting the desired bisect point is implementing one MSR per patch. So, moving the REFERENCE_TSC handling entirely to patch 2 would still be in the "refcount=Y, TSC page=N" case. In any case, this patch needs more comments and a better commit message. Microsoft docs are decent, but there are several non-obvious points in how the patches were done, and they need to be documented.