linux-kernel - Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched clock function

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20190729110927.GC31398@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Jul 2019 13:09:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     lantianyu1986@...il.com, Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        sashal@...nel.org, daniel.lezcano@...aro.org, arnd@...db.de,
        michael.h.kelley@...rosoft.com, ashal@...nel.org
Subject: Re: [PATCH 0/2] clocksource/Hyper-V: Add Hyper-V specific sched
 clock function

On Mon, Jul 29, 2019 at 12:59:26PM +0200, Vitaly Kuznetsov wrote:
> lantianyu1986@...il.com writes:
> 
> > From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> >
> > Hyper-V guests use the default native_sched_clock() in pv_ops.time.sched_clock
> > on x86.  But native_sched_clock() directly uses the raw TSC value, which
> > can be discontinuous in a Hyper-V VM.   Add the generic hv_setup_sched_clock()
> > to set the sched clock function appropriately.  On x86, this sets
> > pv_ops.time.sched_clock to read the Hyper-V reference TSC value that is
> > scaled and adjusted to be continuous.
> 
> Hypervisor can, in theory, disable TSC page and then we're forced to use
> MSR-based clocksource but using it as sched_clock() can be very slow,
> I'm afraid.
> 
> On the other hand, what we have now is probably worse: TSC can,
> actually, jump backwards (e.g. on migration) and we're breaking the
> requirements for sched_clock().

That (obviously) also breaks the requirements for using TSC as
clocksource.

IOW, it breaks the entire purpose of having TSC in the first place.