linux-kernel - Re: [PATCH v10 071/108] KVM: x86: Allow to update cached values in kvm_user_return

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Date:   Mon, 14 Nov 2022 15:36:27 +0800
From:   Binbin Wu <binbin.wu@...ux.intel.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        erdemaktas@...gle.com, Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Chao Gao <chao.gao@...el.com>
Subject: Re: [PATCH v10 071/108] KVM: x86: Allow to update cached values in
 kvm_user_return_msrs w/o wrmsr


On 10/30/2022 2:23 PM, isaku.yamahata@...el.com wrote:
> From: Chao Gao <chao.gao@...el.com>
>
> Several MSRs are constant and only used in userspace(ring 3).  But VMs may
> have different values.  KVM uses kvm_set_user_return_msr() to switch to
> guest's values and leverages user return notifier to restore them when the
> kernel is to return to userspace.  To eliminate unnecessary wrmsr, KVM also
> caches the value it wrote to an MSR last time.
>
> TDX module unconditionally resets some of these MSRs to architectural INIT
> state on TD exit.  It makes the cached values in kvm_user_return_msrs are
> inconsistent with values in hardware.  This inconsistency needs to be
> fixed.  Otherwise, it may mislead kvm_on_user_return() to skip restoring
> some MSRs to the host's values.  kvm_set_user_return_msr() can help correct
> this case, but it is not optimal as it always does a wrmsr.  So, introduce
> a variation of kvm_set_user_return_msr() to update cached values and skip
> that wrmsr.
>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/x86.c              | 26 +++++++++++++++++++++-----
>   2 files changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b225cdfac4bc..fdb00d96e954 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2082,6 +2082,7 @@ int kvm_add_user_return_msr(u32 msr);
>   int kvm_find_user_return_msr(u32 msr);
>   void kvm_user_return_msr_init_cpu(void);
>   int kvm_set_user_return_msr(unsigned index, u64 val, u64 mask);
> +void kvm_user_return_update_cache(unsigned int index, u64 val);
>   
>   static inline bool kvm_is_supported_user_return_msr(u32 msr)
>   {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 65541bfebb37..4d4b71c4cdb1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -440,6 +440,15 @@ void kvm_user_return_msr_init_cpu(void)
>   }
>   EXPORT_SYMBOL_GPL(kvm_user_return_msr_init_cpu);
>   
> +static void kvm_user_return_register_notifier(struct kvm_user_return_msrs *msrs)
> +{
> +	if (!msrs->registered) {
> +		msrs->urn.on_user_return = kvm_on_user_return;
> +		user_return_notifier_register(&msrs->urn);
> +		msrs->registered = true;
> +	}
> +}
> +
>   int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
>   {
>   	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
> @@ -455,15 +464,22 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
>   		return 1;
>   
>   	msrs->values[slot].curr = value;
> -	if (!msrs->registered) {
> -		msrs->urn.on_user_return = kvm_on_user_return;
> -		user_return_notifier_register(&msrs->urn);
> -		msrs->registered = true;
> -	}
> +	kvm_user_return_register_notifier(msrs);
>   	return 0;
>   }
>   EXPORT_SYMBOL_GPL(kvm_set_user_return_msr);
>   
> +/* Update the cache, "curr", and register the notifier */
> +void kvm_user_return_update_cache(unsigned int slot, u64 value)
> +{
> +	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
> +
> +	WARN_ON_ONCE(!msrs->initialized);
> +	msrs->values[slot].curr = value;

If the value is the same as the cached one, can it just skip notifier 
registration?


> +	kvm_user_return_register_notifier(msrs);
> +}
> +EXPORT_SYMBOL_GPL(kvm_user_return_update_cache);
> +
>   static void drop_user_return_notifiers(void)
>   {
>   	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);