From: Sean Christopherson <seanjc@google.com>
To: Peng Hao <flyingpenghao@gmail.com>
Cc: pbonzini@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kvm: x86: Improve virtual machine startup performance
Date: Tue, 1 Mar 2022 17:54:53 +0000 [thread overview]
Message-ID: <Yh5d7XBD9D4FhEe3@google.com> (raw)
In-Reply-To: <20220301063756.16817-1-flyingpeng@tencent.com>
On Tue, Mar 01, 2022, Peng Hao wrote:
> From: Peng Hao <flyingpeng@tencent.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@tencent.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.
next prev parent reply other threads:[~2022-03-01 17:55 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-01 6:37 [PATCH] kvm: x86: Improve virtual machine startup performance Peng Hao
2022-03-01 17:54 ` Sean Christopherson [this message]
2022-03-02 1:30 ` Hao Peng
2022-03-03 1:29 ` Sean Christopherson
2022-03-03 2:56 ` Hao Peng
2022-03-08 2:03 ` Sean Christopherson
2022-03-08 8:13 ` Paolo Bonzini
2022-03-08 16:04 ` Sean Christopherson
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