Re: [PATCH 0/2] RFC: Precise TSC migration

From: Andy Lutomirski <luto@kernel.org>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Oliver Upton <oupton@google.com>, Ingo Molnar <mingo@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	open list <linux-kernel@vger.kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Wanpeng Li <wanpengli@tencent.com>,
	Borislav Petkov <bp@alien8.de>, Jim Mattson <jmattson@google.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Joerg Roedel <joro@8bytes.org>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration
Date: Mon, 30 Nov 2020 08:54:19 -0800	[thread overview]
Message-ID: <CALCETrVr2bM4yJTVpQULN+EYVQJuWGCvjX0SMFsCRy6BwqZc0w@mail.gmail.com> (raw)
In-Reply-To: <20201130133559.233242-1-mlevitsk@redhat.com>

On Mon, Nov 30, 2020 at 5:36 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> Hi!
>
> This is the first version of the work to make TSC migration more accurate,
> as was defined by Paulo at:
> https://www.spinics.net/lists/kvm/msg225525.html
>
> I have a few thoughts about the kvm masterclock synchronization,
> which relate to the Paulo's proposal that I implemented.
>
> The idea of masterclock is that when the host TSC is synchronized
> (or as kernel call it, stable), and the guest TSC is synchronized as well,
> then we can base the kvmclock, on the same pair of
> (host time in nsec, host tsc value), for all vCPUs.
>
> This makes the random error in calculation of this value invariant
> across vCPUS, and allows the guest to do kvmclock calculation in userspace
> (vDSO) since kvmclock parameters are vCPU invariant.
>
> To ensure that the guest tsc is synchronized we currently track host/guest tsc
> writes, and enable the master clock only when roughly the same guest's TSC value
> was written across all vCPUs.
>
> Recently this was disabled by Paulo and I agree with this, because I think
> that we indeed should only make the guest TSC synchronized by default
> (including new hotplugged vCPUs) and not do any tsc synchronization beyond that.
> (Trying to guess when the guest syncs the TSC can cause more harm that good).
>
> Besides, Linux guests don't sync the TSC via IA32_TSC write,
> but rather use IA32_TSC_ADJUST which currently doesn't participate
> in the tsc sync heruistics.
> And as far as I know, Linux guest is the primary (only?) user of the kvmclock.
>
> I *do think* however that we should redefine KVM_CLOCK_TSC_STABLE
> in the documentation to state that it only guarantees invariance if the guest
> doesn't mess with its own TSC.
>
> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE
> in the guest kernel, when kvm is detected to avoid the guest even from trying
> to sync TSC on newly hotplugged vCPUs.
>
> (The guest doesn't end up touching TSC_ADJUST usually, but it still might
> in some cases due to scheduling of guest vCPUs)
>
> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug,
> and TSC clocksource watchdog, and the later we might want to keep).

If you're going to change the guest behavior to be more trusting of
the host, I think
the host should probably signal this to the guest using a new bit.