Re: [PATCH v19 081/130] KVM: x86: Allow to update cached values in kvm_user_return_msrs w/o wrmsr

[Binbin Wu]

On 2/26/2024 4:26 PM, isaku.yamahata@xxxxxxxxx wrote:
> From: Chao Gao <chao.gao@xxxxxxxxx>
>
> 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
                                                                       ^
                                                               extra "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@xxxxxxxxx>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/x86.c              | 25 ++++++++++++++++++++-----
>   2 files changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 36694e784c27..3ab85c3d86ee 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2259,6 +2259,7 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
>   int kvm_add_user_return_msr(u32 msr);
>   int kvm_find_user_return_msr(u32 msr);
>   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 b361d948140f..1b189e86a1f1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -440,6 +440,15 @@ static void kvm_user_return_msr_cpu_online(void)
>   	}
>   }
>   
> +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)
>   {
>   	unsigned int cpu = smp_processor_id();
> @@ -454,15 +463,21 @@ 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 */
Not sure this comment is necessary, since the code is simple.

> +void kvm_user_return_update_cache(unsigned int slot, u64 value)

As a public API, is it better to use "kvm_user_return_msr_update_cache" 
instead of "kvm_user_return_update_cache"?
Although it makes the API name longer...

> +{
> +	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
> +
> +	msrs->values[slot].curr = value;
> +	kvm_user_return_register_notifier(msrs);
> +}
> +EXPORT_SYMBOL_GPL(kvm_user_return_update_cache);
> +
>   static void drop_user_return_notifiers(void)
>   {
>   	unsigned int cpu = smp_processor_id();