Re: [PATCH v2 1/2] KVM: Refactor kvm_arch_vcpu_fault() to return a struct page pointer

[David Hildenbrand <david@xxxxxxxxxx>]

On 12.08.21 06:02, Hou Wenlong wrote:
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> Refactor kvm_arch_vcpu_fault() to return 'struct page *' instead of
> 'vm_fault_t' to simplify architecture specific implementations that do
> more than return SIGBUS.  Currently this only applies to s390, but a
> future patch will move x86's pio_data handling into x86 where it belongs.
>
> No functional changed intended.
>
> Cc: Hou Wenlong <houwenlong93@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Hou Wenlong <houwenlong93@xxxxxxxxxxxxxxxxx>
> ---
>   arch/arm64/kvm/arm.c       |  4 ++--
>   arch/mips/kvm/mips.c       |  4 ++--
>   arch/powerpc/kvm/powerpc.c |  4 ++--
>   arch/s390/kvm/kvm-s390.c   | 12 ++++--------
>   arch/x86/kvm/x86.c         |  4 ++--
>   include/linux/kvm_host.h   |  2 +-
>   virt/kvm/kvm_main.c        |  5 ++++-
>   7 files changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index e9a2b8f27792..83f4ffe3e4f2 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -161,9 +161,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>   	return ret;
>   }
>   
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
>   {
> -	return VM_FAULT_SIGBUS;
> +	return NULL;
>   }
>   
>   
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index af9dd029a4e1..ae79874e6fd2 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -1053,9 +1053,9 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
>   	return -ENOIOCTLCMD;
>   }
>   
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
>   {
> -	return VM_FAULT_SIGBUS;
> +	return NULL;
>   }
>   
>   int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index be33b5321a76..b9c21f9ab784 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -2090,9 +2090,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>   	return r;
>   }
>   
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
>   {
> -	return VM_FAULT_SIGBUS;
> +	return NULL;
>   }
>   
>   static int kvm_vm_ioctl_get_pvinfo(struct kvm_ppc_pvinfo *pvinfo)
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 02574d7b3612..e1b69833e228 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -4979,17 +4979,13 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>   	return r;
>   }
>   
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
>   {
>   #ifdef CONFIG_KVM_S390_UCONTROL
> -	if ((vmf->pgoff == KVM_S390_SIE_PAGE_OFFSET)
> -		 && (kvm_is_ucontrol(vcpu->kvm))) {
> -		vmf->page = virt_to_page(vcpu->arch.sie_block);
> -		get_page(vmf->page);
> -		return 0;
> -	}
> +	if (vmf->pgoff == KVM_S390_SIE_PAGE_OFFSET && kvm_is_ucontrol(vcpu->kvm))
> +		return virt_to_page(vcpu->arch.sie_block);
>   #endif
> -	return VM_FAULT_SIGBUS;
> +	return NULL;
>   }
>   
>   /* Section: memory related */
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3cedc7cc132a..1e3bbe5cd33a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5347,9 +5347,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>   	return r;
>   }
>   
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
>   {
> -	return VM_FAULT_SIGBUS;
> +	return NULL;
>   }
>   
>   static int kvm_vm_ioctl_set_tss_addr(struct kvm *kvm, unsigned long addr)
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 492d183dd7d0..a949de534722 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -995,7 +995,7 @@ long kvm_arch_dev_ioctl(struct file *filp,
>   			unsigned int ioctl, unsigned long arg);
>   long kvm_arch_vcpu_ioctl(struct file *filp,
>   			 unsigned int ioctl, unsigned long arg);
> -vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf);
> +struct page *kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf);
>   
>   int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext);
>   
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 30d322519253..f7d21418971b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3448,7 +3448,10 @@ static vm_fault_t kvm_vcpu_fault(struct vm_fault *vmf)
>   		    &vcpu->dirty_ring,
>   		    vmf->pgoff - KVM_DIRTY_LOG_PAGE_OFFSET);
>   	else
> -		return kvm_arch_vcpu_fault(vcpu, vmf);
> +		page = kvm_arch_vcpu_fault(vcpu, vmf);
> +	if (!page)
> +		return VM_FAULT_SIGBUS;
> +
>   	get_page(page);
>   	vmf->page = page;
>   	return 0;

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

But at the same time I wonder if we should just get rid of 
CONFIG_KVM_S390_UCONTROL and consequently kvm_arch_vcpu_fault().


In practice CONFIG_KVM_S390_UCONTROL, is never enabled in any reasonable 
kernel build and consequently it's never tested; further, exposing the 
sie_block to user space allows user space to generate random SIE 
validity intercepts.

CONFIG_KVM_S390_UCONTROL feels like something that should just be 
maintained out of tree by someone who really needs to hack deep into hw 
virtualization for testing purposes etc.

--
Thanks,

David / dhildenb