Re: [XEN PATCH v2 07/15] x86: guard cpu_has_{svm/vmx} macros with CONFIG_{SVM/VMX}

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.05.2024 11:12, Sergiy Kibrik wrote:
> --- a/xen/arch/x86/include/asm/cpufeature.h
> +++ b/xen/arch/x86/include/asm/cpufeature.h
> @@ -81,7 +81,8 @@ static inline bool boot_cpu_has(unsigned int feat)
>  #define cpu_has_sse3            boot_cpu_has(X86_FEATURE_SSE3)
>  #define cpu_has_pclmulqdq       boot_cpu_has(X86_FEATURE_PCLMULQDQ)
>  #define cpu_has_monitor         boot_cpu_has(X86_FEATURE_MONITOR)
> -#define cpu_has_vmx             boot_cpu_has(X86_FEATURE_VMX)
> +#define cpu_has_vmx             ( IS_ENABLED(CONFIG_VMX) && \
> +                                  boot_cpu_has(X86_FEATURE_VMX))
>  #define cpu_has_eist            boot_cpu_has(X86_FEATURE_EIST)
>  #define cpu_has_ssse3           boot_cpu_has(X86_FEATURE_SSSE3)
>  #define cpu_has_fma             boot_cpu_has(X86_FEATURE_FMA)
> @@ -109,7 +110,8 @@ static inline bool boot_cpu_has(unsigned int feat)
>  
>  /* CPUID level 0x80000001.ecx */
>  #define cpu_has_cmp_legacy      boot_cpu_has(X86_FEATURE_CMP_LEGACY)
> -#define cpu_has_svm             boot_cpu_has(X86_FEATURE_SVM)
> +#define cpu_has_svm             ( IS_ENABLED(CONFIG_SVM) && \
> +                                  boot_cpu_has(X86_FEATURE_SVM))
>  #define cpu_has_sse4a           boot_cpu_has(X86_FEATURE_SSE4A)
>  #define cpu_has_xop             boot_cpu_has(X86_FEATURE_XOP)
>  #define cpu_has_skinit          boot_cpu_has(X86_FEATURE_SKINIT)

Hmm, leaving aside the style issue (stray blanks after opening parentheses,
and as a result one-off indentation on the wrapped lines) I'm not really
certain we can do this. The description goes into detail why we would want
this, but it doesn't cover at all why it is safe for all present (and
ideally also future) uses. I wouldn't be surprised if we had VMX/SVM checks
just to derive further knowledge from that, without them being directly
related to the use of VMX/SVM. Take a look at calculate_hvm_max_policy(),
for example. While it looks to be okay there, it may give you an idea of
what I mean.

Things might become better separated if instead for such checks we used
host and raw CPU policies instead of cpuinfo_x86.x86_capability[]. But
that's still pretty far out, I'm afraid.

Jan