Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Message-ID: <2f1459d7c3e3e81cdca931e104c3ade71dfcfee5.camel@infradead.org>
Subject: Re: [PATCH RFC 1/1] x86/paravirt: introduce param to disable pv
 sched_clock
From:   David Woodhouse <dwmw2@infradead.org>
To:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Dongli Zhang <dongli.zhang@oracle.com>, x86@kernel.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        pv-drivers@vmware.com, xen-devel@lists.xenproject.org,
        linux-hyperv@vger.kernel.org, jgross@suse.com, akaher@vmware.com,
        amakhalov@vmware.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
        pbonzini@redhat.com, wanpengli@tencent.com, peterz@infradead.org,
        joe.jin@oracle.com, boris.ostrovsky@oracle.com,
        linux-kernel@vger.kernel.org
Date:   Thu, 19 Oct 2023 16:47:05 +0100
In-Reply-To: <ZTFOCqMCuSiH8VEt@google.com>
References: <20231018221123.136403-1-dongli.zhang@oracle.com>
         <87ttqm6d3f.fsf@redhat.com> <ZTFOCqMCuSiH8VEt@google.com>
Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature";
        boundary="=-IOFzLyxqUftWQ1c88CUC"
User-Agent: Evolution 3.44.4-0ubuntu2 
MIME-Version: 1.0
Precedence: bulk


--=-IOFzLyxqUftWQ1c88CUC
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Thu, 2023-10-19 at 08:40 -0700, Sean Christopherson wrote:
>=20
> > Normally, it should be up to the hypervisor to tell the guest which
> > clock to use, i.e. if TSC is reliable or not. Let me put my question
> > this way: if TSC on the particular host is good for everything, why
> > does the hypervisor advertises 'kvmclock' to its guests?
>=20
> I suspect there are two reasons.
>=20
> =C2=A0 1. As is likely the case in our fleet, no one revisited the set of=
 advertised
> =C2=A0=C2=A0=C2=A0=C2=A0 PV features when defining the VM shapes for a ne=
w generation of hardware, or
> =C2=A0=C2=A0=C2=A0=C2=A0 whoever did the reviews wasn't aware that advert=
ising kvmclock is actually
> =C2=A0=C2=A0=C2=A0=C2=A0 suboptimal.=C2=A0 All the PV clock stuff in KVM =
is quite labyrinthian, so it's
> =C2=A0=C2=A0=C2=A0=C2=A0 not hard to imagine it getting overlooked.
>=20
> =C2=A0 2. Legacy VMs.=C2=A0 If VMs have been running with a PV clock for =
years, forcing
> =C2=A0=C2=A0=C2=A0=C2=A0 them to switch to a new clocksource is high-risk=
, low-reward.

Doubly true for Xen guests (given that the Xen clocksource is identical
to the KVM clocksource).

> > If for some 'historical reasons' we can't revoke features we can always
> > introduce a new PV feature bit saying that TSC is preferred.

Don't we already have one? It's the PVCLOCK_TSC_STABLE_BIT. Why would a
guest ever use kvmclock if the PVCLOCK_TSC_STABLE_BIT is set?

The *point* in the kvmclock is that the hypervisor can mess with the
epoch/scaling to try to compensate for TSC brokenness as the host
scales/sleeps/etc.

And the *problem* with the kvmclock is that it does just that, even
when the host TSC hasn't done anything wrong and the kvmclock shouldn't
have changed at all.

If the PVCLOCK_TSC_STABLE_BIT is set, a guest should just use the guest
TSC directly without looking to the kvmclock for adjusting it.

No?


--=-IOFzLyxqUftWQ1c88CUC
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Disposition: attachment; filename="smime.p7s"
Content-Transfer-Encoding: base64
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--=-IOFzLyxqUftWQ1c88CUC--