On Tue, Mar 01, 2022, Peng Hao wrote:
> From: Peng Hao <flyingpeng@xxxxxxxxxxx>
>
> vcpu 0 will repeatedly enter/exit the smm state during the startup
> phase, and kvm_init_mmu will be called repeatedly during this process.
> There are parts of the mmu initialization code that do not need to be
> modified after the first initialization.
>
> Statistics on my server, vcpu0 when starting the virtual machine
> Calling kvm_init_mmu more than 600 times (due to smm state switching).
> The patch can save about 36 microseconds in total.
>
> Signed-off-by: Peng Hao <flyingpeng@xxxxxxxxxxx>
> ---
> @@ -5054,7 +5059,7 @@ void kvm_mmu_after_set_cpuid(struct kvm_vcpu *vcpu)
> void kvm_mmu_reset_context(struct kvm_vcpu *vcpu)
> {
> kvm_mmu_unload(vcpu);
> - kvm_init_mmu(vcpu);
> + kvm_init_mmu(vcpu, false);
This is wrong, kvm_mmu_reset_context() is the "big hammer" and is expected to
unconditionally get the MMU to a known good state. E.g. failure to initialize
means this code:
context->shadow_root_level = kvm_mmu_get_tdp_level(vcpu);
will not update the shadow_root_level as expected in response to userspace changing
guest.MAXPHYADDR in such a way that KVM enables/disables 5-level paging.
The SMM transitions definitely need to be fixed, and we're slowly getting there,
but sadly there's no quick fix.
