From: Paolo Bonzini <pbonzini@redhat.com> To: Marcelo Tosatti <mtosatti@redhat.com> Cc: Eduardo Habkost <ehabkost@redhat.com>, kvm@vger.kernel.org, Juan Quintela <quintela@redhat.com>, Radim Krcmar <rkrcmar@redhat.com>, qemu-devel@nongnu.org, "Dr. David Alan Gilbert" <dgilbert@redhat.com> Subject: Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration Date: Mon, 14 Nov 2016 16:00:38 +0100 [thread overview] Message-ID: <67bffd95-2e4e-7273-c154-a3fdfe622387@redhat.com> (raw) In-Reply-To: <20161114145054.GA28663@amt.cnet> On 14/11/2016 15:50, Marcelo Tosatti wrote: > Well, i didnt want to mix the meaning of the variables: > > + /* whether machine supports reliable KVM_GET_CLOCK */ > + bool mach_use_reliable_get_clock; > + > + /* whether source host supported reliable KVM_GET_CLOCK */ > + bool src_use_reliable_get_clock; > > See the comments on top (later if you look at the variable, > then have to think: well it has one name, but its disabled > by that other path as well, so its more than its > name,etc...). > >> I'm thinking "mach_use_reliable_get_clock is just for migration, > > Thats whether the machine supports it. New machines have it enabled, > olders don't. Yes. >> src_use_reliable_get_clock is the state". > > Thats whether the migration source supported it. But it's not used only for migration. It's used on every vmstate change (running->stop and stop->running, isn't it? I think that, apart from the migration case, it's better to use s->clock if kvmclock is stable, even on older machine types. >>>>> +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->src_use_reliable_get_clock; >>>>> +} >>>> >>>> Here you can just return s->mach_use_reliable_get_clock. >>> >>> mach_use_reliable_get_clock can be true but host might not support it. >> >> Yes, but the "needed" function is only required to avoid breaking >> pc-i440fx-2.7 and earlier. > > "needed" is required so that the migration between: > > SRC DEST BEHAVIOUR > ~support supports on migration read from guest, > on stop/cont use > kvm_get_clock/kvm_set_clock > > Destination does not use KVM_GET_CLOCK value (which is > broken and should not be used). If needed returns false, the destination will see src_use_reliable_get_clock = false anyway. If needed returns true, the destination can still see src_use_reliable_get_clock = false if that's the value. needed src_use_reliable_get_clock effect false false use kvmclock_current_nsec false true use kvmclock_current_nsec true false use kvmclock_current_nsec true true use s->clock So the idea is: - set s->clock and s->reliable_get_clock on every KVM_GET_CLOCK - on migration source, use subsections and s->mach_use_reliable_get_clock to avoid breaking migration on pre-2.8 machine types - on migration destination, use .pre_load so that s->reliable_get_clock is initialized to false on older machine types >> If you return true here, you can still >> migrate a "false" value for src_use_reliable_get_clock. > > But the source only uses a reliable KVM_GET_CLOCK if > both conditions are true. > > And the subsection is only needed if the source > uses a reliable KVM_GET_CLOCK. Yes, but the point of the subsection is just to avoid breaking migration format. >> It is the same as "is using masterclock", which is actually a stricter >> condition than the KVM_CHECK_EXTENSION return value. The right check to >> use is whether masterclock is in use, > > Actually its "has a reliable KVM_GET_CLOCK" (which returns > get_kernel_clock() + (rdtsc() - tsc_timestamp), > > "broken KVM_GET_CLOCK" = get_kernel_clock() Yes. And these end up being the same. Paolo
WARNING: multiple messages have this Message-ID (diff)
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-devel] [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration Date: Mon, 14 Nov 2016 16:00:38 +0100 [thread overview] Message-ID: <67bffd95-2e4e-7273-c154-a3fdfe622387@redhat.com> (raw) In-Reply-To: <20161114145054.GA28663@amt.cnet> On 14/11/2016 15:50, Marcelo Tosatti wrote: > Well, i didnt want to mix the meaning of the variables: > > + /* whether machine supports reliable KVM_GET_CLOCK */ > + bool mach_use_reliable_get_clock; > + > + /* whether source host supported reliable KVM_GET_CLOCK */ > + bool src_use_reliable_get_clock; > > See the comments on top (later if you look at the variable, > then have to think: well it has one name, but its disabled > by that other path as well, so its more than its > name,etc...). > >> I'm thinking "mach_use_reliable_get_clock is just for migration, > > Thats whether the machine supports it. New machines have it enabled, > olders don't. Yes. >> src_use_reliable_get_clock is the state". > > Thats whether the migration source supported it. But it's not used only for migration. It's used on every vmstate change (running->stop and stop->running, isn't it? I think that, apart from the migration case, it's better to use s->clock if kvmclock is stable, even on older machine types. >>>>> +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->src_use_reliable_get_clock; >>>>> +} >>>> >>>> Here you can just return s->mach_use_reliable_get_clock. >>> >>> mach_use_reliable_get_clock can be true but host might not support it. >> >> Yes, but the "needed" function is only required to avoid breaking >> pc-i440fx-2.7 and earlier. > > "needed" is required so that the migration between: > > SRC DEST BEHAVIOUR > ~support supports on migration read from guest, > on stop/cont use > kvm_get_clock/kvm_set_clock > > Destination does not use KVM_GET_CLOCK value (which is > broken and should not be used). If needed returns false, the destination will see src_use_reliable_get_clock = false anyway. If needed returns true, the destination can still see src_use_reliable_get_clock = false if that's the value. needed src_use_reliable_get_clock effect false false use kvmclock_current_nsec false true use kvmclock_current_nsec true false use kvmclock_current_nsec true true use s->clock So the idea is: - set s->clock and s->reliable_get_clock on every KVM_GET_CLOCK - on migration source, use subsections and s->mach_use_reliable_get_clock to avoid breaking migration on pre-2.8 machine types - on migration destination, use .pre_load so that s->reliable_get_clock is initialized to false on older machine types >> If you return true here, you can still >> migrate a "false" value for src_use_reliable_get_clock. > > But the source only uses a reliable KVM_GET_CLOCK if > both conditions are true. > > And the subsection is only needed if the source > uses a reliable KVM_GET_CLOCK. Yes, but the point of the subsection is just to avoid breaking migration format. >> It is the same as "is using masterclock", which is actually a stricter >> condition than the KVM_CHECK_EXTENSION return value. The right check to >> use is whether masterclock is in use, > > Actually its "has a reliable KVM_GET_CLOCK" (which returns > get_kernel_clock() + (rdtsc() - tsc_timestamp), > > "broken KVM_GET_CLOCK" = get_kernel_clock() Yes. And these end up being the same. Paolo
next prev parent reply other threads:[~2016-11-14 15:00 UTC|newest] Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-11-14 12:36 [qemu patch 0/2] improve kvmclock difference on migration Marcelo Tosatti 2016-11-14 12:36 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 12:36 ` [qemu patch 1/2] kvm: sync linux headers Marcelo Tosatti 2016-11-14 12:36 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 12:36 ` [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration Marcelo Tosatti 2016-11-14 12:36 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 13:54 ` Paolo Bonzini 2016-11-14 13:54 ` [Qemu-devel] " Paolo Bonzini 2016-11-14 14:00 ` Marcelo Tosatti 2016-11-14 14:00 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 14:22 ` Paolo Bonzini 2016-11-14 14:22 ` [Qemu-devel] " Paolo Bonzini 2016-11-14 14:50 ` Marcelo Tosatti 2016-11-14 14:50 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 15:00 ` Paolo Bonzini [this message] 2016-11-14 15:00 ` Paolo Bonzini 2016-11-14 15:40 ` Marcelo Tosatti 2016-11-14 15:40 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 16:43 ` Paolo Bonzini 2016-11-14 16:43 ` [Qemu-devel] " Paolo Bonzini 2016-11-14 17:13 ` Marcelo Tosatti 2016-11-14 17:13 ` [Qemu-devel] " Marcelo Tosatti 2016-11-14 17:20 ` Paolo Bonzini 2016-11-14 17:20 ` [Qemu-devel] " Paolo Bonzini 2016-11-14 18:15 ` Marcelo Tosatti 2016-11-14 18:15 ` [Qemu-devel] " Marcelo Tosatti 2016-11-17 12:16 ` Marcelo Tosatti 2016-11-17 12:16 ` [Qemu-devel] " Marcelo Tosatti 2016-11-17 13:03 ` Paolo Bonzini 2016-11-17 13:03 ` [Qemu-devel] " Paolo Bonzini 2016-11-28 13:47 ` Paolo Bonzini 2016-11-28 13:47 ` [Qemu-devel] " Paolo Bonzini 2016-11-28 14:28 ` Eduardo Habkost 2016-11-28 14:28 ` [Qemu-devel] " Eduardo Habkost 2016-11-28 15:12 ` Paolo Bonzini 2016-11-28 15:12 ` [Qemu-devel] " Paolo Bonzini 2016-11-28 16:36 ` Marcelo Tosatti 2016-11-28 16:36 ` [Qemu-devel] " Marcelo Tosatti 2016-11-28 17:30 ` Paolo Bonzini 2016-11-28 17:30 ` [Qemu-devel] " Paolo Bonzini 2016-11-14 14:11 ` Juan Quintela 2016-11-14 14:11 ` [Qemu-devel] " Juan Quintela 2016-11-14 14:09 ` Juan Quintela 2016-11-14 14:09 ` [Qemu-devel] " Juan Quintela 2016-11-14 15:37 ` Marcelo Tosatti 2016-11-14 15:37 ` [Qemu-devel] " Marcelo Tosatti
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