[PATCH v1 0/4] KVM: s390: Implement KVM_PRE_FAULT_MEMORY

[PATCH v1 0/4] KVM: s390: Implement KVM_PRE_FAULT_MEMORY

[Claudio Imbrenda]

Implement KVM_PRE_FAULT_MEMORY on s390, and update documentation and
selftests accordingly.

Faulted-in pages will be marked as accessed, unlike x86, otherwise they
will trigger a minor fault when accessed. Avoiding such faults is one of
the points of KVM_PRE_FAULT_MEMORY.

CCing x86 people due to changes in the documentation and the selftests

Claudio Imbrenda (4):
  KVM: s390: Implement KVM_PRE_FAULT_MEMORY
  KVM: s390: Update KVM_PRE_FAULT_MEMORY API documentation
  KVM: selftests: Fix pre_fault_memory_test to run on s390
  KVM: selftests: Enable pre_fault_memory_test for s390

 Documentation/virt/kvm/api.rst                |  5 ++-
 arch/s390/kvm/Kconfig                         |  1 +
 arch/s390/kvm/kvm-s390.c                      | 41 +++++++++++++++++++
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../selftests/kvm/pre_fault_memory_test.c     |  7 ++--
 5 files changed, 50 insertions(+), 5 deletions(-)

-- 
2.54.0

[PATCH v1 1/4] KVM: s390: Implement KVM_PRE_FAULT_MEMORY

[Claudio Imbrenda]

Implement and enable the KVM_PRE_FAULT_MEMORY ioctl for s390.

Faulted-in pages will be marked as accessed, unlike x86, otherwise they
will trigger a minor fault when accessed. Avoiding such faults is one of
the points of KVM_PRE_FAULT_MEMORY.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/Kconfig    |  1 +
 arch/s390/kvm/kvm-s390.c | 41 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/arch/s390/kvm/Kconfig b/arch/s390/kvm/Kconfig
index 5b835bc6a194..8d3ee17a1bcb 100644
--- a/arch/s390/kvm/Kconfig
+++ b/arch/s390/kvm/Kconfig
@@ -30,6 +30,7 @@ config KVM
 	select KVM_VFIO
 	select VIRT_XFER_TO_GUEST_WORK
 	select KVM_MMU_LOCKLESS_AGING
+	select KVM_GENERIC_PRE_FAULT_MEMORY
 	help
 	  Support hosting paravirtualized guest machines using the SIE
 	  virtualization capability on the mainframe. This should work
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index e09960c2e6ed..dd220f3d6e94 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -630,6 +630,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_S390_USER_OPEREXEC:
 	case KVM_CAP_S390_KEYOP:
 	case KVM_CAP_S390_VSIE_ESAMODE:
+	case KVM_CAP_PRE_FAULT_MEMORY:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
@@ -5736,6 +5737,46 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 	return;
 }
 
+/**
+ * kvm_arch_vcpu_pre_fault_memory() -- pre-fault and link gmap dat tables
+ * @vcpu: the vcpu that shall appear to have generated the fault-in.
+ * @range: the range that needs to be faulted in.
+ *
+ * The given range is faulted in and the corresponding gmap page tables are
+ * created, as if the given vCPU had performed a read operation. If the range
+ * ends outside of a memslot, only the part inside the memslot is faulted in.
+ * If the range starts outside any memslots, an error is returned. An error is
+ * also returned for UCONTROL VMs, which should instead use the
+ * KVM_S390_VCPU_FAULT ioctl.
+ *
+ * Return:
+ * * %-ENOENT if the range lies outside of a memslot.
+ * * %-EINVAL in case of invalid state (for example if the VM is UCONTROL)
+ * * %-EIO if other errors happen while faulting-in the page (considered a
+ *   bug, will trigger a warning in the caller).
+ * * other error codes < 0 in case of other errors.
+ * * otherwise a number > 0 of bytes that have been faulted in successfully.
+ */
+long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu, struct kvm_pre_fault_memory *range)
+{
+	unsigned long i;
+	int rc;
+
+	if (kvm_is_ucontrol(vcpu->kvm))
+		return -EINVAL;
+
+	for (i = 0; i < range->size; i += PAGE_SIZE) {
+		rc = kvm_s390_faultin_gfn_simple(vcpu, NULL, gpa_to_gfn(range->gpa + i), false);
+		if (rc == PGM_ADDRESSING)
+			return i ? i : -ENOENT;
+		if (rc == -EINTR)
+			return i ? i : -EINTR;
+		if (rc)
+			return -EIO;
+	}
+	return i;
+}
+
 /**
  * kvm_test_age_gfn() - test young
  * @kvm: the kvm instance
-- 
2.54.0

[PATCH v1 2/4] KVM: s390: Update KVM_PRE_FAULT_MEMORY API documentation

[Claudio Imbrenda]

Update the API documentation for KVM_PRE_FAULT_MEMORY to account for
its s390 implementation.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 Documentation/virt/kvm/api.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 52bbbb553ce1..e7998feaa940 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6471,7 +6471,8 @@ Errors:
 
   ========== ===============================================================
   EINVAL     The specified `gpa` and `size` were invalid (e.g. not
-             page aligned, causes an overflow, or size is zero).
+             page aligned, causes an overflow, or size is zero), or the VM
+             is UCONTROL (s390).
   ENOENT     The specified `gpa` is outside defined memslots.
   EINTR      An unmasked signal is pending and no page was processed.
   EFAULT     The parameter address was invalid.
@@ -6494,7 +6495,7 @@ Errors:
 KVM_PRE_FAULT_MEMORY populates KVM's stage-2 page tables used to map memory
 for the current vCPU state.  KVM maps memory as if the vCPU generated a
 stage-2 read page fault, e.g. faults in memory as needed, but doesn't break
-CoW.  However, KVM does not mark any newly created stage-2 PTE as Accessed.
+CoW.  On x86, KVM does not mark any newly created stage-2 PTE as Accessed.
 
 In the case of confidential VM types where there is an initial set up of
 private guest memory before the guest is 'finalized'/measured, this ioctl
-- 
2.54.0

[PATCH v1 3/4] KVM: selftests: Fix pre_fault_memory_test to run on s390

[Claudio Imbrenda]

Add a missing #include <ucall_common.h> which is needed and otherwise
not included on s390.

Fence the assertion  vcpu->run->exit_reason == KVM_EXIT_IO  so that it
is only checked on x86. On s390 the UCALL will return with a different
code.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 tools/testing/selftests/kvm/pre_fault_memory_test.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/pre_fault_memory_test.c b/tools/testing/selftests/kvm/pre_fault_memory_test.c
index fcb57fd034e6..42794892e902 100644
--- a/tools/testing/selftests/kvm/pre_fault_memory_test.c
+++ b/tools/testing/selftests/kvm/pre_fault_memory_test.c
@@ -11,6 +11,7 @@
 #include <kvm_util.h>
 #include <processor.h>
 #include <pthread.h>
+#include <ucall_common.h>
 
 /* Arbitrarily chosen values */
 #define TEST_SIZE		(SZ_2M + PAGE_SIZE)
@@ -167,7 +168,6 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
 		.type = vm_type,
 	};
 	struct kvm_vcpu *vcpu;
-	struct kvm_run *run;
 	struct kvm_vm *vm;
 	struct ucall uc;
 
@@ -193,10 +193,11 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
 	vcpu_args_set(vcpu, 1, gva);
 	vcpu_run(vcpu);
 
-	run = vcpu->run;
-	TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
+#ifdef __x86_64__
+	TEST_ASSERT(vcpu->run->exit_reason == KVM_EXIT_IO,
 		    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
 		    run->exit_reason, exit_reason_str(run->exit_reason));
+#endif
 
 	switch (get_ucall(vcpu, &uc)) {
 	case UCALL_ABORT:
-- 
2.54.0

[PATCH v1 4/4] KVM: selftests: Enable pre_fault_memory_test for s390

[Claudio Imbrenda]

Enable the pre_fault_memory_test to run on s390.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 tools/testing/selftests/kvm/Makefile.kvm | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 9118a5a51b89..fff939db89cd 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -210,6 +210,7 @@ TEST_GEN_PROGS_s390 += s390/keyop
 TEST_GEN_PROGS_s390 += rseq_test
 TEST_GEN_PROGS_s390 += s390/irq_routing
 TEST_GEN_PROGS_s390 += mmu_stress_test
+TEST_GEN_PROGS_s390 += pre_fault_memory_test
 
 TEST_GEN_PROGS_riscv = $(TEST_GEN_PROGS_COMMON)
 TEST_GEN_PROGS_riscv += riscv/sbi_pmu_test
-- 
2.54.0

Re: [PATCH v1 3/4] KVM: selftests: Fix pre_fault_memory_test to run on s390

[Sean Christopherson]

On Wed, May 20, 2026, Claudio Imbrenda wrote:
> Add a missing #include <ucall_common.h> which is needed and otherwise
> not included on s390.
> 
> Fence the assertion  vcpu->run->exit_reason == KVM_EXIT_IO  so that it
> is only checked on x86. On s390 the UCALL will return with a different
> code.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
>  tools/testing/selftests/kvm/pre_fault_memory_test.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/pre_fault_memory_test.c b/tools/testing/selftests/kvm/pre_fault_memory_test.c
> index fcb57fd034e6..42794892e902 100644
> --- a/tools/testing/selftests/kvm/pre_fault_memory_test.c
> +++ b/tools/testing/selftests/kvm/pre_fault_memory_test.c
> @@ -11,6 +11,7 @@
>  #include <kvm_util.h>
>  #include <processor.h>
>  #include <pthread.h>
> +#include <ucall_common.h>
>  
>  /* Arbitrarily chosen values */
>  #define TEST_SIZE		(SZ_2M + PAGE_SIZE)
> @@ -167,7 +168,6 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
>  		.type = vm_type,
>  	};
>  	struct kvm_vcpu *vcpu;
> -	struct kvm_run *run;
>  	struct kvm_vm *vm;
>  	struct ucall uc;
>  
> @@ -193,10 +193,11 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
>  	vcpu_args_set(vcpu, 1, gva);
>  	vcpu_run(vcpu);
>  
> -	run = vcpu->run;
> -	TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
> +#ifdef __x86_64__

I'd say just delete this assertion entirely.  It's an extra layer of paranoia to
guard against stale ucall data, but x86's ucall_arch_get_ucall() already has
sufficient guards.

> +	TEST_ASSERT(vcpu->run->exit_reason == KVM_EXIT_IO,
>  		    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
>  		    run->exit_reason, exit_reason_str(run->exit_reason));
> +#endif
>  
>  	switch (get_ucall(vcpu, &uc)) {
>  	case UCALL_ABORT:
> -- 
> 2.54.0
> 

Re: [PATCH v1 3/4] KVM: selftests: Fix pre_fault_memory_test to run on s390

[Claudio Imbrenda]

On Wed, 20 May 2026 08:27:24 -0700
Sean Christopherson <seanjc@google.com> wrote:

> On Wed, May 20, 2026, Claudio Imbrenda wrote:
> > Add a missing #include <ucall_common.h> which is needed and otherwise
> > not included on s390.
> > 
> > Fence the assertion  vcpu->run->exit_reason == KVM_EXIT_IO  so that it
> > is only checked on x86. On s390 the UCALL will return with a different
> > code.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> > ---
> >  tools/testing/selftests/kvm/pre_fault_memory_test.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/kvm/pre_fault_memory_test.c b/tools/testing/selftests/kvm/pre_fault_memory_test.c
> > index fcb57fd034e6..42794892e902 100644
> > --- a/tools/testing/selftests/kvm/pre_fault_memory_test.c
> > +++ b/tools/testing/selftests/kvm/pre_fault_memory_test.c
> > @@ -11,6 +11,7 @@
> >  #include <kvm_util.h>
> >  #include <processor.h>
> >  #include <pthread.h>
> > +#include <ucall_common.h>
> >  
> >  /* Arbitrarily chosen values */
> >  #define TEST_SIZE		(SZ_2M + PAGE_SIZE)
> > @@ -167,7 +168,6 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
> >  		.type = vm_type,
> >  	};
> >  	struct kvm_vcpu *vcpu;
> > -	struct kvm_run *run;
> >  	struct kvm_vm *vm;
> >  	struct ucall uc;
> >  
> > @@ -193,10 +193,11 @@ static void __test_pre_fault_memory(unsigned long vm_type, bool private)
> >  	vcpu_args_set(vcpu, 1, gva);
> >  	vcpu_run(vcpu);
> >  
> > -	run = vcpu->run;
> > -	TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
> > +#ifdef __x86_64__  
> 
> I'd say just delete this assertion entirely.  It's an extra layer of paranoia to
> guard against stale ucall data, but x86's ucall_arch_get_ucall() already has
> sufficient guards.

will do, thanks!

> 
> > +	TEST_ASSERT(vcpu->run->exit_reason == KVM_EXIT_IO,
> >  		    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
> >  		    run->exit_reason, exit_reason_str(run->exit_reason));
> > +#endif
> >  
> >  	switch (get_ucall(vcpu, &uc)) {
> >  	case UCALL_ABORT:
> > -- 
> > 2.54.0
> >