linux-kernel - Re: [PATCH v3 01/22] KVM: x86: Drop kvm_user_return_msr_cpu

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Date:   Mon, 5 Sep 2022 13:30:38 +0800
From:   Yuan Yao <yuan.yao@...ux.intel.com>
To:     isaku.yamahata@...el.com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        isaku.yamahata@...il.com, Kai Huang <kai.huang@...el.com>,
        Chao Gao <chao.gao@...el.com>,
        Atish Patra <atishp@...shpatra.org>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Qi Liu <liuqi115@...wei.com>,
        John Garry <john.garry@...wei.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Huang Ying <ying.huang@...el.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v3 01/22] KVM: x86: Drop kvm_user_return_msr_cpu_online()


On Thu, Sep 01, 2022 at 07:17:36PM -0700, isaku.yamahata@...el.com wrote:
> From: Sean Christopherson <seanjc@...gle.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.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>

Reviewed-by: Yuan Yao <yuan.yao@...el.com>

> ---
>  arch/x86/kvm/x86.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 205ebdc2b11b..0e200fe44b35 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -196,6 +196,7 @@ module_param(eager_page_split, bool, 0644);
>
>  struct kvm_user_return_msrs {
>  	struct user_return_notifier urn;
> +	bool initialized;
>  	bool registered;
>  	struct kvm_user_return_msr_values {
>  		u64 host;
> @@ -409,18 +410,20 @@ int kvm_find_user_return_msr(u32 msr)
>  }
>  EXPORT_SYMBOL_GPL(kvm_find_user_return_msr);
>
> -static void kvm_user_return_msr_cpu_online(void)
> +static void kvm_user_return_msr_init_cpu(struct kvm_user_return_msrs *msrs)
>  {
> -	unsigned int cpu = smp_processor_id();
> -	struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
>  	u64 value;
>  	int i;
>
> +	if (msrs->initialized)
> +		return;
> +
>  	for (i = 0; i < kvm_nr_uret_msrs; ++i) {
>  		rdmsrl_safe(kvm_uret_msrs_list[i], &value);
>  		msrs->values[i].host = value;
>  		msrs->values[i].curr = value;
>  	}
> +	msrs->initialized = true;
>  }
>
>  int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
> @@ -429,6 +432,8 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
>  	struct kvm_user_return_msrs *msrs = per_cpu_ptr(user_return_msrs, cpu);
>  	int err;
>
> +	kvm_user_return_msr_init_cpu(msrs);
> +
>  	value = (value & mask) | (msrs->values[slot].host & ~mask);
>  	if (value == msrs->values[slot].curr)
>  		return 0;
> @@ -9212,7 +9217,12 @@ int kvm_arch_init(void *opaque)
>  		return -ENOMEM;
>  	}
>
> -	user_return_msrs = alloc_percpu(struct kvm_user_return_msrs);
> +	/*
> +	 * __GFP_ZERO to ensure user_return_msrs.values[].initialized = false.
> +	 * See kvm_user_return_msr_init_cpu().
> +	 */
> +	user_return_msrs = alloc_percpu_gfp(struct kvm_user_return_msrs,
> +					    GFP_KERNEL | __GFP_ZERO);
>  	if (!user_return_msrs) {
>  		printk(KERN_ERR "kvm: failed to allocate percpu kvm_user_return_msrs\n");
>  		r = -ENOMEM;
> @@ -11836,7 +11846,6 @@ int kvm_arch_hardware_enable(void)
>  	u64 max_tsc = 0;
>  	bool stable, backwards_tsc = false;
>
> -	kvm_user_return_msr_cpu_online();
>  	ret = static_call(kvm_x86_hardware_enable)();
>  	if (ret != 0)
>  		return ret;
> --
> 2.25.1
>