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

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Date:   Tue, 1 Mar 2022 17:54:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peng Hao <flyingpenghao@...il.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH]  kvm: x86: Improve virtual machine startup performance

On Tue, Mar 01, 2022, Peng Hao wrote:
>  From: Peng Hao <flyingpeng@...cent.com>
> 
> 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@...cent.com>
> ---
> @@ -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.