Re: [PATCH] kvm: x86: Improve virtual machine startup performance

[Hao Peng]

On Wed, Mar 2, 2022 at 1:54 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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.
>
Thanks for pointing this out. However, other than shadow_root_level,
other fields of context will not
change during the entire operation, such as
page_fault/sync_page/direct_map and so on under
the condition of tdp_mmu.
Is this patch still viable after careful confirmation of the fields
that won't be modified?
thanks.
> The SMM transitions definitely need to be fixed, and we're slowly getting there,
> but sadly there's no quick fix.