Re: [PATCH 11/13] KVM: SVM: Drop redundant svm_sev_enabled() helper

[Tom Lendacky]

On 1/8/21 6:47 PM, Sean Christopherson wrote:
> Replace calls to svm_sev_enabled() with direct checks on sev_enabled, or
> in the case of svm_mem_enc_op, simply drop the call to svm_sev_enabled().
> This effectively replaces checks against a valid max_sev_asid with checks
> against sev_enabled.  sev_enabled is forced off by sev_hardware_setup()
> if max_sev_asid is invalid, all call sites are guaranteed to run after
> sev_hardware_setup(), and all of the checks care about SEV being fully
> enabled (as opposed to intentionally handling the scenario where
> max_sev_asid is valid but SEV enabling fails due to OOM).
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/svm/sev.c | 6 +++---
>   arch/x86/kvm/svm/svm.h | 5 -----
>   2 files changed, 3 insertions(+), 8 deletions(-)
> 

With CONFIG_KVM=y, CONFIG_KVM_AMD=y and CONFIG_CRYPTO_DEV_CCP_DD=m, I get
the following build warning:

make: Entering directory '/root/kernels/kvm-build-x86_64'
  DESCEND  objtool               
  CALL    scripts/atomic/check-atomics.sh
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h                                                           
  CC      arch/x86/kvm/svm/svm.o                                                                                                                                                                
  CC      arch/x86/kvm/svm/nested.o                                                             
  CC      arch/x86/kvm/svm/avic.o                                                                                                                                                               
  CC      arch/x86/kvm/svm/sev.o          
In file included from ./include/linux/cpumask.h:12,     
                 from ./arch/x86/include/asm/cpumask.h:5,
                 from ./arch/x86/include/asm/msr.h:11,
                 from ./arch/x86/include/asm/processor.h:22,
                 from ./arch/x86/include/asm/cpufeature.h:5,                                                                                                                                    
                 from ./arch/x86/include/asm/thread_info.h:53,
                 from ./include/linux/thread_info.h:38,
                 from ./arch/x86/include/asm/preempt.h:7,
                 from ./include/linux/preempt.h:78,
                 from ./include/linux/percpu.h:6,
                 from ./include/linux/context_tracking_state.h:5,
                 from ./include/linux/hardirq.h:5,
                 from ./include/linux/kvm_host.h:7,
                 from arch/x86/kvm/svm/sev.c:11:
In function ‘bitmap_zero’,
    inlined from ‘__sev_recycle_asids’ at arch/x86/kvm/svm/sev.c:92:2,
    inlined from ‘sev_asid_new’ at arch/x86/kvm/svm/sev.c:113:16,
    inlined from ‘sev_guest_init’ at arch/x86/kvm/svm/sev.c:195:9:
./include/linux/bitmap.h:238:2: warning: argument 1 null where non-null expected [-Wnonnull]
  238 |  memset(dst, 0, len);
      |  ^~~~~~~~~~~~~~~~~~~
In file included from ./arch/x86/include/asm/string.h:5,
                 from ./include/linux/string.h:20,
                 from ./include/linux/bitmap.h:9,
                 from ./include/linux/cpumask.h:12,
                 from ./arch/x86/include/asm/cpumask.h:5,
                 from ./arch/x86/include/asm/msr.h:11,
                 from ./arch/x86/include/asm/processor.h:22,
                 from ./arch/x86/include/asm/cpufeature.h:5,
                 from ./arch/x86/include/asm/thread_info.h:53,
                 from ./include/linux/thread_info.h:38,
                 from ./arch/x86/include/asm/preempt.h:7,
                 from ./include/linux/preempt.h:78,
                 from ./include/linux/percpu.h:6,
                 from ./include/linux/context_tracking_state.h:5,
                 from ./include/linux/hardirq.h:5,
                 from ./include/linux/kvm_host.h:7,
                 from arch/x86/kvm/svm/sev.c:11:
arch/x86/kvm/svm/sev.c: In function ‘sev_guest_init’:
./arch/x86/include/asm/string_64.h:18:7: note: in a call to function ‘memset’ declared here
   18 | void *memset(void *s, int c, size_t n);
      |       ^~~~~~

Thanks,
Tom

> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 8c34c467a09d..1b9174a49b65 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1052,7 +1052,7 @@ int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
>   	struct kvm_sev_cmd sev_cmd;
>   	int r;
>   
> -	if (!svm_sev_enabled() || !sev_enabled)
> +	if (!sev_enabled)
>   		return -ENOTTY;
>   
>   	if (!argp)
> @@ -1314,7 +1314,7 @@ void __init sev_hardware_setup(void)
>   
>   void sev_hardware_teardown(void)
>   {
> -	if (!svm_sev_enabled())
> +	if (!sev_enabled)
>   		return;
>   
>   	bitmap_free(sev_asid_bitmap);
> @@ -1325,7 +1325,7 @@ void sev_hardware_teardown(void)
>   
>   int sev_cpu_init(struct svm_cpu_data *sd)
>   {
> -	if (!svm_sev_enabled())
> +	if (!sev_enabled)
>   		return 0;
>   
>   	sd->sev_vmcbs = kmalloc_array(max_sev_asid + 1, sizeof(void *),
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 4eb4bab0ca3e..8cb4395b58a0 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -569,11 +569,6 @@ void svm_vcpu_unblocking(struct kvm_vcpu *vcpu);
>   
>   extern unsigned int max_sev_asid;
>   
> -static inline bool svm_sev_enabled(void)
> -{
> -	return IS_ENABLED(CONFIG_KVM_AMD_SEV) ? max_sev_asid : 0;
> -}
> -
>   void sev_vm_destroy(struct kvm *kvm);
>   int svm_mem_enc_op(struct kvm *kvm, void __user *argp);
>   int svm_register_enc_region(struct kvm *kvm,
>