On 07/09/2019 08:19 PM, Peter Zijlstra wrote:
For the lbr feature, could we thought of it as first come, first served?
For example, if we have 2 host threads who want to use lbr at the same time,
I think one of them would simply fail to use.
So if guest first gets the lbr, host wouldn't take over unless some
userspace command (we added to QEMU) is executed to have the vCPU
actively stop using lbr.
Doesn't work that way.
Say you start KVM with LBR emulation, it creates this task event, it
gets the LBR (nobody else wants it) and the guest works and starts using
the LBR.
Then the host creates a CPU LBR event and the vCPU suddenly gets denied
the LBR and the guest no longer functions correctly.
Or you should fail to VMENTER, in which case you starve the guest, but
at least it doesn't malfunction.
Ok, I think we could go with the first option for now, like this:
When there are other host threads starting to use lbr, host perf notifies
kvm to disable the guest use of lbr (add a notifier_block), which includes:
- cancel passing through lbr msrs to guest
- upon guest access to the lbr msr (will trap to KVM now), return #GP
(or 0).
Best,
Wei
