From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support\ Date: Wed, 3 Oct 2018 16:05:04 -0300 Message-ID: <20181003190500.GA23638@amt.cnet> References: <20180914125006.349747096@linutronix.de> <20181003190026.GB21381@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20181003190026.GB21381@amt.cnet> 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: Andy Lutomirski Cc: Wanpeng Li , Florian Weimer , Juergen Gross , Arnd Bergmann , Radim Krcmar , Peter Zijlstra , X86 ML , LKML , Linux Virtualization , Stephen Boyd , John Stultz , devel@linuxdriverproject.org, Paolo Bonzini , Thomas Gleixner , Matt Rickard List-Id: virtualization@lists.linuxfoundation.org On Wed, Oct 03, 2018 at 04:00:29PM -0300, Marcelo Tosatti wrote: > On Tue, Oct 02, 2018 at 10:15:49PM -0700, Andy Lutomirski wrote: > > Hi Vitaly, Paolo, Radim, etc., > > > > On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner wrote: > > > > > > Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime() > > > implementation, which extended the clockid switch case and added yet > > > another slightly different copy of the same code. > > > > > > Especially the extended switch case is problematic as the compiler tends to > > > generate a jump table which then requires to use retpolines. If jump tables > > > are disabled it adds yet another conditional to the existing maze. > > > > > > This series takes a different approach by consolidating the almost > > > identical functions into one implementation for high resolution clocks and > > > one for the coarse grained clock ids by storing the base data for each > > > clock id in an array which is indexed by the clock id. > > > > > > > I was trying to understand more of the implications of this patch > > series, and I was again reminded that there is an entire extra copy of > > the vclock reading code in arch/x86/kvm/x86.c. And the purpose of > > that code is very, very opaque. > > > > Can one of you explain what the code is even doing? From a couple of > > attempts to read through it, it's a whole bunch of > > probably-extremely-buggy code that, > > Yes, probably. > > > drumroll please, tries to atomically read the TSC value and the time. And decide whether the > > result is "based on the TSC". > > I think "based on the TSC" refers to whether TSC clocksource is being > used. > > > And then synthesizes a TSC-to-ns > > multiplier and shift, based on *something other than the actual > > multiply and shift used*. > > > > IOW, unless I'm totally misunderstanding it, the code digs into the > > private arch clocksource data intended for the vDSO, uses a poorly > > maintained copy of the vDSO code to read the time (instead of doing > > the sane thing and using the kernel interfaces for this), and > > propagates a totally made up copy to the guest. > > I posted kernel interfaces for this, and it was suggested to > instead write a "in-kernel user of pvclock data". > > If you can get kernel interfaces to replace that, go for it. I prefer > kernel interfaces as well. And cleanup patches, to make that code look nicer, are also very welcome!