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Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <1832e2af-3189-cb50-f4b6-45e74cdcf4b4@redhat.com>
Date:   Thu, 4 Oct 2018 07:04:38 -0700
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Florian Weimer <fweimer@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
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References: <20180914125006.349747096@linutronix.de> <CALCETrU+jP2hPLoJWTYJKTvr7-=YLwtk1=cZ_uizHYQZ1z4P-w@mail.gmail.com> <87sh1ne64t.fsf@vitty.brq.redhat.com> <20181003190617.GC21381@amt.cnet> <87k1mycfju.fsf@vitty.brq.redhat.com> <1832e2af-3189-cb50-f4b6-45e74cdcf4b4@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Oct 4, 2018, at 5:00 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>=20
>> On 04/10/2018 09:54, Vitaly Kuznetsov wrote:
>> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling
>> is supported).
>=20
> Not if you want to migrate to pre-Skylake systems.
>=20
>> - Check if non-masterclock mode is still needed. E.g. HyperV's TSC page
>> clocksource is a single page for the whole VM, not a per-cpu thing. Can
>> we think that all the buggy hardware is already gone?
>=20
> No. :(  We still get reports whenever we break 2007-2008 hardware.
>=20
>=20

Does the KVM non-masterclock mode actually help?  It=E2=80=99s not clear to m=
e exactly how it=E2=80=99s supposed to work, but it seems like it=E2=80=99s t=
rying to expose per-vCPU adjustments to the guest. Which is dubious at best,=
 since the guest can=E2=80=99t validly use them for anything other than sche=
d_clock, since they aren=E2=80=99t fully corrected by anything KVM can do.=