Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

From: Andy Lutomirski <luto@kernel.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Maxim Levitsky <mlevitsk@redhat.com>,
	kvm list <kvm@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Jim Mattson <jmattson@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	open list <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Borislav Petkov <bp@alien8.de>, Shuah Khan <shuah@kernel.org>,
	Andrew Jones <drjones@redhat.com>,
	Oliver Upton <oupton@google.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Date: Tue, 15 Dec 2020 08:55:15 -0800	[thread overview]
Message-ID: <CALCETrWSnMm-mfC5LjzsFLk6Tt_40Udrif-Kh34TTj6fp_8ZLw@mail.gmail.com> (raw)
In-Reply-To: <20201215105927.GA3321@fuller.cnet>

On Tue, Dec 15, 2020 at 3:35 AM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Fri, Dec 11, 2020 at 10:59:59PM +0100, Paolo Bonzini wrote:
> > On 11/12/20 22:04, Thomas Gleixner wrote:
> > > > Its 100ms off with migration, and can be reduced further (customers
> > > > complained about 5 seconds but seem happy with 0.1ms).
> > > What is 100ms? Guaranteed maximum migration time?
> >
> > I suppose it's the length between the time from KVM_GET_CLOCK and
> > KVM_GET_MSR(IA32_TSC) to KVM_SET_CLOCK and KVM_SET_MSR(IA32_TSC).  But the
> > VM is paused for much longer, the sequence for the non-live part of the
> > migration (aka brownout) is as follows:
> >
> >     pause
> >     finish sending RAM            receive RAM               ~1 sec
> >     send paused-VM state          finish receiving RAM     \
> >                                   receive paused-VM state   ) 0.1 sec
> >                                   restart                  /
> >
> > The nanosecond and TSC times are sent as part of the paused-VM state at the
> > very end of the live migration process.
> >
> > So it's still true that the time advances during live migration brownout;
> > 0.1 seconds is just the final part of the live migration process.  But for
> > _live_ migration there is no need to design things according to "people are
> > happy if their clock is off by 0.1 seconds only".
>
> Agree. What would be a good way to fix this?
>

Could you implement the Hyper-V clock interface?  It's much, much
simpler than the kvmclock interface.  It has the downside that
CLOCK_BOOTTIME won't do what you want, but I'm not really convinced
that's a problem, and you could come up with a minimal extension to
fix that.