Re: tlb_flush stat on Intel/AMD

[Jacob Xu <jacobhxu@google.com>]

> The VMX APIC flush could be deferred by hoisting KVM_REQ_APIC_PAGE_RELOAD
> up.  I think that's safe?  But it's a very infrequent operation so I'm not
> exactly chomping at the bit to get it fixed.

Could moving KVM_REQ_TLB_FLUSH and _CURRENT down also work?
It seems that none of the check_request() calls below depend on them.

> Given that you see 0 on SVM and a low number on VMX, my money is on the
> difference being that VMX accounts the TLB flush that occurs on vCPU
> migration.  vmx_vcpu_load_vmcs() makes a KVM_REQ_TLB_FLUSH request, whereas
> svm_vcpu_load() resets asid_generation but doesn't increment the stats.

I'll try adding a one-off stat increment here, and check the results.

> I think a better option is to keep the current accounting, defer flushes
> when possible to naturally fix accounting, and then fix the remaining one
> off cases, e.g. kvm_mmu_load() and svm_vcpu_load().
>
> I can prep a small series unless you want the honors?

I'll try making a small series:
kvm: selftests: add get_debugfs_stat to kvm_util
kvm: x86: increment tlb_flush stat on svm_vcpu_load(), kvm_mmu_load()
# omit kvm_mmu_load() if feasible to move check_request(KVM_REQ_TLB_FLUSH) down
kvm: x86: re-order check_request() to prefer use of deferred tlb_flush

Thanks for the explanations! Sorry for my late reply, I've updated my
email filter accordingly.