Re: [PATCH] KVM: MMU: Add wrapper to check whether MMU is in direct mode

[Sean Christopherson <seanjc@google.com>]

+David and Ben

On Tue, Dec 06, 2022, Yu Zhang wrote:
> Simplify the code by introducing a wrapper, mmu_is_direct(),
> instead of using vcpu->arch.mmu->root_role.direct everywhere.
> 
> Meanwhile, use temporary variable 'direct', in routines such
> as kvm_mmu_load()/kvm_mmu_page_fault() etc. instead of checking
> vcpu->arch.mmu->root_role.direct repeatedly.

I've looked at this patch at least four times and still can't decide whether or
not I like the helper.  On one had, it's shorter and easier to read.  On the other
hand, I don't love that mmu_is_nested() looks at a completely different MMU, which
is weird if not confusing.

Anyone else have an opinion?

> No functional change intended.
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 26 +++++++++++++-------------
>  arch/x86/kvm/x86.c     |  9 +++++----
>  arch/x86/kvm/x86.h     |  5 +++++
>  3 files changed, 23 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4736d7849c60..d2d0fabdb702 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2280,7 +2280,7 @@ static void shadow_walk_init_using_root(struct kvm_shadow_walk_iterator *iterato
>  
>  	if (iterator->level >= PT64_ROOT_4LEVEL &&
>  	    vcpu->arch.mmu->cpu_role.base.level < PT64_ROOT_4LEVEL &&
> -	    !vcpu->arch.mmu->root_role.direct)
> +	    !mmu_is_direct(vcpu))
>  		iterator->level = PT32E_ROOT_LEVEL;
>  
>  	if (iterator->level == PT32E_ROOT_LEVEL) {
> @@ -2677,7 +2677,7 @@ static int kvm_mmu_unprotect_page_virt(struct kvm_vcpu *vcpu, gva_t gva)
>  	gpa_t gpa;
>  	int r;
>  
> -	if (vcpu->arch.mmu->root_role.direct)
> +	if (mmu_is_direct(vcpu))
>  		return 0;
>  
>  	gpa = kvm_mmu_gva_to_gpa_read(vcpu, gva, NULL);
> @@ -3918,7 +3918,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
>  	int i;
>  	struct kvm_mmu_page *sp;
>  
> -	if (vcpu->arch.mmu->root_role.direct)
> +	if (mmu_is_direct(vcpu))
>  		return;
>  
>  	if (!VALID_PAGE(vcpu->arch.mmu->root.hpa))
> @@ -4147,7 +4147,7 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  
>  	arch.token = alloc_apf_token(vcpu);
>  	arch.gfn = gfn;
> -	arch.direct_map = vcpu->arch.mmu->root_role.direct;
> +	arch.direct_map = mmu_is_direct(vcpu);
>  	arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);
>  
>  	return kvm_setup_async_pf(vcpu, cr2_or_gpa,
> @@ -4157,17 +4157,16 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>  {
>  	int r;
> +	bool direct = mmu_is_direct(vcpu);

I would prefer to not add local bools and instead due a 1:1 replacement.  "direct"
loses too much context (direct what?), and performance wise I doubt it will
influence the compiler.