Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration

From: Paolo Bonzini <pbonzini@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm@vger.kernel.org, qemu-devel@nongnu.org,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	Radim Krcmar <rkrcmar@redhat.com>,
	Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration
Date: Mon, 14 Nov 2016 17:43:33 +0100	[thread overview]
Message-ID: <cbcac3d2-88aa-3cb4-7257-e70061a22a2c@redhat.com> (raw)
In-Reply-To: <20161114154015.GA30048@amt.cnet>

On 14/11/2016 16:40, Marcelo Tosatti wrote:
> static bool kvmclock_src_use_reliable_get_clock(void *opaque)
> {
>     KVMClockState *s = opaque;
> 
>     /*
>      * On machine types that support reliable KVM_GET_CLOCK,
>      * if host kernel does provide reliable KVM_GET_CLOCK,
>      * set src_use_reliable_get_clock=true so that destination
>      * avoids reading kvmclock from memory.
>      */
>     if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable())
>     {
>         s->src_use_reliable_get_clock = true;
>     }
> 
>     return s->mach_use_reliable_get_clock;
> }
> 
> 
> Ah, OK, done.

s->src_use_reliable_get_clock should not be set with
KVM_CHECK_EXTENSION, but rather from the flags returned by KVM_GET_CLOCK.

> So s->src_use_reliable_get_clock is only used to indicate 
> to the destination that: "you can use KVM_GET_CLOCK value, 
> its safe".

Yes, we agree.  I was listing all the points, not just those where we
disagree.  Actually I'm not sure where we disagree, except on using
flags from KVM_CHECK_EXTENSION vs. flags from KVM_GET_CLOCK...

Paolo