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
