Re: [PATCH v3 03/15] KVM: arm64: x86: Require "struct kvm_page_fault" for memory fault exits

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Jun 18, 2025 at 04:24:12AM +0000, James Houghton wrote:
> +#ifdef CONFIG_KVM_GENERIC_PAGE_FAULT
> +
> +#define KVM_ASSERT_TYPE_IS(type_t, x)					\
> +do {									\
> +	type_t __maybe_unused tmp;					\
> +									\
> +	BUILD_BUG_ON(!__types_ok(tmp, x) || !__typecheck(tmp, x));	\
> +} while (0)
> +
>  static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> -						 gpa_t gpa, gpa_t size,
> -						 bool is_write, bool is_exec,
> -						 bool is_private)
> +						 struct kvm_page_fault *fault)
>  {
> +	KVM_ASSERT_TYPE_IS(gfn_t, fault->gfn);
> +	KVM_ASSERT_TYPE_IS(bool, fault->exec);
> +	KVM_ASSERT_TYPE_IS(bool, fault->write);
> +	KVM_ASSERT_TYPE_IS(bool, fault->is_private);
> +	KVM_ASSERT_TYPE_IS(struct kvm_memory_slot *, fault->slot);
> +
>  	vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> -	vcpu->run->memory_fault.gpa = gpa;
> -	vcpu->run->memory_fault.size = size;
> +	vcpu->run->memory_fault.gpa = fault->gfn << PAGE_SHIFT;
> +	vcpu->run->memory_fault.size = PAGE_SIZE;
>  
>  	/* RWX flags are not (yet) defined or communicated to userspace. */
>  	vcpu->run->memory_fault.flags = 0;
> -	if (is_private)
> +	if (fault->is_private)
>  		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
>  }
> +#endif

This *is not* the right direction of travel for arm64. Stage-2 aborts /
EPT violations / etc. are extremely architecture-specific events.

What I would like to see on arm64 is that for every "KVM_EXIT_MEMORY_FAULT"
we provide as much syndrome information as possible. That could imply
some combination of a sanitised view of ESR_EL2 and, where it is
unambiguous, common fault flags that have shared definitions with x86.
This could incur some minor code duplication, but even then we can share
helpers for the software bits (like userfault).

FEAT_MTE_PERM is a very good example for this. There exists a "Tag"
permission at stage-2 which is unrelated to any of the 'normal'
read/write permissions. There's also the MostlyReadOnly permission from
FEAT_THE which grants write permission to a specific set of instructions.

I don't want to paper over these nuances and will happily maintain an
arm64-specific flavor of "kvm_prepare_memory_fault_exit()"/

Thanks,
Oliver