linux-kernel - Re: [PATCH v2 01/19] KVM: x86: Drop kvm_user_return_msr_cpu

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Date:   Thu, 1 Sep 2022 13:29:41 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     <isaku.yamahata@...el.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <isaku.yamahata@...il.com>, Paolo Bonzini <pbonzini@...hat.com>,
        "Sean Christopherson" <seanjc@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        "Will Deacon" <will@...nel.org>
Subject: Re: [PATCH v2 01/19] KVM: x86: Drop kvm_user_return_msr_cpu_online()

On Tue, Aug 30, 2022 at 05:01:16AM -0700, isaku.yamahata@...el.com wrote:
>From: Isaku Yamahata <isaku.yamahata@...el.com>
>
>KVM/X86 uses user return notifier to switch MSR for guest or user space.
>Snapshot host values on CPU online, change MSR values for guest, and
>restore them on returning to user space.  The current code abuses
>kvm_arch_hardware_enable() which is called on kvm module initialization or
>CPU online.
>
>Remove such the abuse of kvm_arch_hardware_enable by capturing the host
>value on the first change of the MSR value to guest VM instead of CPU
>online.
>
>Suggested-by: Sean Christopherson <seanjc@...gle.com>
>Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
>---
> arch/x86/kvm/x86.c | 43 ++++++++++++++++++++++++-------------------
> 1 file changed, 24 insertions(+), 19 deletions(-)
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 205ebdc2b11b..16104a2f7d8e 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -200,6 +200,7 @@ struct kvm_user_return_msrs {
> 	struct kvm_user_return_msr_values {
> 		u64 host;
> 		u64 curr;
>+		bool initialized;
> 	} values[KVM_MAX_NR_USER_RETURN_MSRS];

The benefit of having an "initialized" state for each user return MSR on
each CPU is small. A per-cpu state looks suffice. With it, you can keep
kvm_user_return_msr_cpu_online() and simply call the function from
kvm_set_user_return_msr() if initialized is false on current CPU.