Re: [PATCH 08/17] Add clock sync request to hardware enable

From: Avi Kivity
Date: Tue Jun 15 2010 - 04:25:01 EST

On 06/15/2010 10:34 AM, Zachary Amsden wrote:
If there are active VCPUs which are marked as belonging to
a particular hardware CPU, request a clock sync for them when
enabling hardware; the TSC could be desynchronized on a newly
arriving CPU, and we need to recompute guests system time
relative to boot after a suspend event.

This covers both cases.

Note that it is acceptable to take the spinlock, as either
no other tasks will be running and no locks held (BSP after
resume), or other tasks will be guaranteed to drop the lock
relatively quickly (AP on CPU_STARTING).

Noting we now get clock synchronization requests for VCPUs
which are starting up (or restarting), it is tempting to
attempt to remove the arch/x86/kvm/x86.c CPU hot-notifiers
at this time, however it is not correct to do so; they are
required for systems with non-constant TSC as the frequency
may not be known immediately after the processor has started
until the cpufreq driver has had a chance to run and query
the chipset.

Updated: implement better locking semantics for hardware_enable

Removed the hack of dropping and retaking the lock by adding the
semantic that we always hold kvm_lock when hardware_enable is
called. The one place that doesn't need to worry about it is
resume, as resuming a frozen CPU, the spinlock won't be taken.

Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
arch/x86/kvm/x86.c | 8 ++++++++
virt/kvm/kvm_main.c | 6 +++++-
2 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4b15d03..05c559d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5442,7 +5442,15 @@ int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)

int kvm_arch_hardware_enable(void *garbage)
+ struct kvm *kvm;
+ struct kvm_vcpu *vcpu;
+ int i;
+ list_for_each_entry(kvm,&vm_list, vm_list)
+ kvm_for_each_vcpu(i, vcpu, kvm)
+ if (vcpu->cpu == smp_processor_id())
+ kvm_request_guest_time_update(vcpu);
return kvm_x86_ops->hardware_enable(garbage);

An alternative to this loop (and a similar one in the cpu frequency notifier earlier) is to have a per-cpu cpu_freq_generation_counter, which is checked on every entry against a snapshot of the counter in the vcpu. This would replace the loop with an O(1) mechanism, at the cost of another compare.

I don't think it's worthwhile at this point though, just something to keep in mind.

error compiling committee.c: too many arguments to function

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