From: Marcelo Tosatti <mtosatti@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
Glauber Costa <glommer@parallels.com>,
kvm@vger.kernel.org, johnstul@us.ibm.com, zamsden@gmail.com,
gleb@redhat.com, pbonzini@redhat.com
Subject: Re: [patch 09/18] KVM: x86: introduce facility to support vsyscall pvclock, via MSR
Date: Wed, 31 Oct 2012 01:12:19 -0200 [thread overview]
Message-ID: <20121031031219.GA9913@amt.cnet> (raw)
In-Reply-To: <508FA054.6080207@redhat.com>
On Tue, Oct 30, 2012 at 11:39:32AM +0200, Avi Kivity wrote:
> On 10/29/2012 08:40 PM, Marcelo Tosatti wrote:
> > On Mon, Oct 29, 2012 at 10:44:41AM -0700, Jeremy Fitzhardinge wrote:
> >> On 10/29/2012 07:45 AM, Glauber Costa wrote:
> >> > On 10/24/2012 05:13 PM, Marcelo Tosatti wrote:
> >> >> Allow a guest to register a second location for the VCPU time info
> >> >>
> >> >> structure for each vcpu (as described by MSR_KVM_SYSTEM_TIME_NEW).
> >> >> This is intended to allow the guest kernel to map this information
> >> >> into a usermode accessible page, so that usermode can efficiently
> >> >> calculate system time from the TSC without having to make a syscall.
> >> >>
> >> >> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >> > Can you please be a bit more specific about why we need this? Why does
> >> > the host need to provide us with two pages with the exact same data? Why
> >> > can't just do it with mapping tricks in the guest?
> >>
> >> In Xen the pvclock structure is embedded within a pile of other stuff
> >> that shouldn't be mapped into guest memory, so providing for a second
> >> location allows it to be placed whereever is convenient for the guest.
> >> That's a restriction of the Xen ABI, but I don't know if it affects KVM.
> >>
> >> J
> >
> > It is possible to share the data for KVM in theory, but:
> >
> > - It is a small amount of memory.
> > - It requires aligning to page size (the in-kernel percpu array
> > is currently cacheline aligned).
> > - It is possible to modify flags separately for userspace/kernelspace,
> > if desired.
> >
> > This justifies the duplication IMO (code is simple and clean).
> >
>
> What would be the changes required to remove the duplication? If it's
> just page alignment, then is seems even smaller. In addition we avoid
> expanding the ABI again.
This would require changing the kernel copy from percpu data, which
there is no guarantee is linear (necessary for fixmap mapping), to
dynamically allocated (which in turn can be tricky due to early boot
clock requirement).
Hum, no thanks.
next prev parent reply other threads:[~2012-10-31 3:27 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-24 13:13 [patch 00/18] pvclock vsyscall support + KVM hypervisor support (v2) Marcelo Tosatti
2012-10-24 13:13 ` [patch 01/18] KVM: x86: retain pvclock guest stopped bit in guest memory Marcelo Tosatti
2012-10-24 13:13 ` [patch 02/18] x86: pvclock: make sure rdtsc doesnt speculate out of region Marcelo Tosatti
2012-10-24 13:13 ` [patch 03/18] x86: pvclock: remove pvclock_shadow_time Marcelo Tosatti
2012-10-30 9:23 ` Avi Kivity
2012-10-30 9:24 ` Avi Kivity
2012-10-24 13:13 ` [patch 04/18] x86: pvclock: create helper for pvclock data retrieval Marcelo Tosatti
2012-10-24 13:13 ` [patch 05/18] x86: pvclock: fix flags usage race Marcelo Tosatti
2012-10-24 13:13 ` [patch 06/18] x86: pvclock: introduce helper to read flags Marcelo Tosatti
2012-10-24 13:13 ` [patch 07/18] sched: add notifier for cross-cpu migrations Marcelo Tosatti
2012-10-24 13:13 ` [patch 08/18] x86: pvclock: generic pvclock vsyscall initialization Marcelo Tosatti
2012-10-29 14:18 ` Glauber Costa
2012-10-29 14:54 ` Marcelo Tosatti
2012-10-29 17:46 ` Jeremy Fitzhardinge
2012-10-29 14:39 ` Glauber Costa
2012-10-24 13:13 ` [patch 09/18] KVM: x86: introduce facility to support vsyscall pvclock, via MSR Marcelo Tosatti
2012-10-29 14:45 ` Glauber Costa
2012-10-29 17:44 ` Jeremy Fitzhardinge
2012-10-29 18:40 ` Marcelo Tosatti
2012-10-30 7:41 ` Glauber Costa
2012-10-30 9:39 ` Avi Kivity
2012-10-31 3:12 ` Marcelo Tosatti [this message]
2012-11-02 10:21 ` Glauber Costa
2012-10-30 7:38 ` Glauber Costa
2012-10-24 13:13 ` [patch 10/18] x86: kvm guest: pvclock vsyscall support Marcelo Tosatti
2012-10-24 13:13 ` [patch 11/18] x86: vsyscall: pass mode to gettime backend Marcelo Tosatti
2012-10-29 14:47 ` Glauber Costa
2012-10-29 18:41 ` Marcelo Tosatti
2012-10-30 7:42 ` Glauber Costa
2012-10-24 13:13 ` [patch 12/18] x86: vdso: pvclock gettime support Marcelo Tosatti
2012-10-29 14:59 ` Glauber Costa
2012-10-29 18:42 ` Marcelo Tosatti
2012-10-30 7:49 ` Glauber Costa
2012-10-31 3:16 ` Marcelo Tosatti
2012-10-24 13:13 ` [patch 13/18] KVM: x86: pass host_tsc to read_l1_tsc Marcelo Tosatti
2012-10-29 15:04 ` Glauber Costa
2012-10-29 18:45 ` Marcelo Tosatti
2012-10-30 7:55 ` Glauber Costa
2012-10-24 13:13 ` [patch 14/18] time: export time information for KVM pvclock Marcelo Tosatti
2012-11-10 1:02 ` John Stultz
2012-11-13 21:07 ` Marcelo Tosatti
2012-10-24 13:13 ` [patch 15/18] KVM: x86: implement PVCLOCK_TSC_STABLE_BIT pvclock flag Marcelo Tosatti
2012-10-30 8:34 ` Glauber Costa
2012-10-31 3:19 ` [patch 15/18] KVM: x86: implement PVCLOCK_TSC_STABLE_BIT pvclock flag\ Marcelo Tosatti
2012-10-24 13:13 ` [patch 16/18] KVM: x86: notifier for clocksource changes Marcelo Tosatti
2012-10-24 13:13 ` [patch 17/18] KVM: x86: add kvm_arch_vcpu_postcreate callback, move TSC initialization Marcelo Tosatti
2012-10-24 13:13 ` [patch 18/18] KVM: x86: require matched TSC offsets for master clock Marcelo Tosatti
2012-10-31 22:46 ` [patch 00/16] pvclock vsyscall support + KVM hypervisor support (v3) Marcelo Tosatti
2012-10-31 22:46 ` [patch 01/16] KVM: x86: retain pvclock guest stopped bit in guest memory Marcelo Tosatti
2012-11-01 10:39 ` Gleb Natapov
2012-11-01 20:51 ` Marcelo Tosatti
2012-11-01 13:44 ` Glauber Costa
2012-10-31 22:46 ` [patch 02/16] x86: pvclock: make sure rdtsc doesnt speculate out of region Marcelo Tosatti
2012-11-01 11:48 ` Gleb Natapov
2012-11-01 13:49 ` Glauber Costa
2012-11-01 13:51 ` Gleb Natapov
2012-11-01 20:56 ` Marcelo Tosatti
2012-11-01 22:13 ` Gleb Natapov
2012-11-01 22:21 ` Marcelo Tosatti
2012-11-02 6:02 ` Gleb Natapov
2012-10-31 22:46 ` [patch 03/16] x86: pvclock: remove pvclock_shadow_time Marcelo Tosatti
2012-11-01 13:52 ` Glauber Costa
2012-10-31 22:47 ` [patch 04/16] x86: pvclock: create helper for pvclock data retrieval Marcelo Tosatti
2012-11-01 14:04 ` Glauber Costa
2012-11-01 20:57 ` Marcelo Tosatti
2012-10-31 22:47 ` [patch 05/16] x86: pvclock: introduce helper to read flags Marcelo Tosatti
2012-11-01 14:07 ` Glauber Costa
2012-11-01 21:08 ` Marcelo Tosatti
2012-10-31 22:47 ` [patch 06/16] sched: add notifier for cross-cpu migrations Marcelo Tosatti
2012-11-01 14:08 ` Glauber Costa
2012-10-31 22:47 ` [patch 07/16] x86: pvclock: generic pvclock vsyscall initialization Marcelo Tosatti
2012-11-01 14:19 ` Glauber Costa
2012-10-31 22:47 ` [patch 08/16] KVM: x86: introduce facility to support vsyscall pvclock, via MSR Marcelo Tosatti
2012-11-01 14:28 ` Glauber Costa
2012-11-01 21:39 ` Marcelo Tosatti
2012-11-02 10:23 ` Glauber Costa
2012-11-02 13:00 ` Marcelo Tosatti
2012-11-05 8:03 ` Glauber Costa
2012-10-31 22:47 ` [patch 09/16] x86: kvm guest: pvclock vsyscall support Marcelo Tosatti
2012-11-02 9:42 ` Glauber Costa
2012-11-05 8:35 ` Marcelo Tosatti
2012-10-31 22:47 ` [patch 10/16] x86: vdso: pvclock gettime support Marcelo Tosatti
2012-11-01 14:41 ` Glauber Costa
2012-11-01 21:42 ` Marcelo Tosatti
2012-11-02 0:33 ` Marcelo Tosatti
2012-11-02 10:25 ` Glauber Costa
2012-11-14 10:42 ` Gleb Natapov
2012-11-14 22:42 ` Marcelo Tosatti
2012-10-31 22:47 ` [patch 11/16] KVM: x86: pass host_tsc to read_l1_tsc Marcelo Tosatti
2012-10-31 22:47 ` [patch 12/16] time: export time information for KVM pvclock Marcelo Tosatti
2012-10-31 22:47 ` [patch 13/16] KVM: x86: implement PVCLOCK_TSC_STABLE_BIT pvclock flag Marcelo Tosatti
2012-10-31 22:47 ` [patch 14/16] KVM: x86: notifier for clocksource changes Marcelo Tosatti
2012-10-31 22:47 ` [patch 15/16] KVM: x86: add kvm_arch_vcpu_postcreate callback, move TSC initialization Marcelo Tosatti
2012-10-31 22:47 ` [patch 16/16] KVM: x86: require matched TSC offsets for master clock Marcelo Tosatti
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