Re: [PATCH 1/2] KVM: x86: reduce pvclock_gtod_sync_lock critical sections

[Paolo Bonzini]

On 08/04/21 14:00, Marcelo Tosatti wrote:
> > KVM_REQ_MCLOCK_INPROGRESS is only needed to kick running vCPUs out of the
> > execution loop;
> We do not want vcpus with different system_timestamp/tsc_timestamp
> pair:
>
>   * To avoid that problem, do not allow visibility of distinct
>   * system_timestamp/tsc_timestamp values simultaneously: use a master
>   * copy of host monotonic time values. Update that master copy
>   * in lockstep.
>
> So KVM_REQ_MCLOCK_INPROGRESS also ensures that no vcpu enters
> guest mode (via vcpu->requests check before VM-entry) with a
> different system_timestamp/tsc_timestamp pair.

Yes this is what KVM_REQ_MCLOCK_INPROGRESS does, but it does not have to 
be done that way.  All you really need is the IPI with KVM_REQUEST_WAIT, 
which ensures that updates happen after the vCPUs have exited guest 
mode.  You don't need to loop on vcpu->requests for example, because 
kvm_guest_time_update could just spin on pvclock_gtod_sync_lock until 
pvclock_update_vm_gtod_copy is done.

So this morning I tried protecting the kvm->arch fields for kvmclock 
using a seqcount, which is nice also because get_kvmclock_ns() does not 
have to bounce the cacheline of pvclock_gtod_sync_lock anymore.  I'll 
post it tomorrow or next week.

Paolo