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

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 12.08.21 11:04, David Hildenbrand wrote:
> On 12.08.21 06:02, Hou Wenlong wrote:
>> From: Sean Christopherson <seanjc@google.com>
>>
>> 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@linux.alibaba.com>
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Hou Wenlong <houwenlong93@linux.alibaba.com>
>> ---
>>   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@redhat.com>
> 
> 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.

I recently talked to the ucontrol users and they will look into selftests.