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

Christian Borntraeger <borntraeger@xxxxxxxxxx> · Mon, 23 Aug 2021 16:12:40 +0200

On 12.08.21 11:04, David Hildenbrand wrote:
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.

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