[PATCH v3 0/5] KVM: s390: Implement KVM_PRE_FAULT_MEMORY

[PATCH v3 0/5] 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.

This series depends on some kvm/s390 functions that are slightly buggy
at the moment. Those functions are being fixed in a separate series.
Even without those fixes, this series is not buggy, it's just a bit
inefficient in some corner cases.

CCing x86 people due to changes in the documentation and the selftests
just for completeness, since the selftest has been acked already.

v2->v3:
* Fix typos
* Correctly indicate page size in gmap_handle_minor_crste_fault() also
  in case nothing needed to be done.
* Pick up and acked-by

v1->v2:
* Add the crste_region3 flag in struct guest_fault to keep track of the
  page size used to solve a fault.
* Rewrite the s390 version of kvm_arch_vcpu_pre_fault_memory() so it
  looks and behaves similarly to the x86 version. No more loops, better
  handling of hugepage mappings, and more correct error handling.
* Remove the assertion vcpu->run->exit_reason == KVM_EXIT_IO from the
  selftest, as it is redundant and x86-specific.

Claudio Imbrenda (5):
  KVM: s390: Track page size in struct guest_fault
  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/dat.h                           |  1 +
 arch/s390/kvm/gmap.c                          |  2 +
 arch/s390/kvm/kvm-s390.c                      | 45 +++++++++++++++++++
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../selftests/kvm/pre_fault_memory_test.c     |  7 +--
 7 files changed, 54 insertions(+), 8 deletions(-)

-- 
2.54.0

[PATCH v3 1/5] KVM: s390: Track page size in struct guest_fault

[Claudio Imbrenda]

Until now, the members of struct guest_fault are always accessed while
holding the required locks, and thus the ptep and crstep pointers can
be dereferenced safely.

There will be some new cases where callers of kvm_s390_faultin_gfn()
need to know the size of the page used to solve the fault, at which
point no locks are held anymore, and dereferencing the crstep field
is not possible.

Introduce a new crste_region3 flag for struct guest_fault to indicate
whether the crstep used to solve the fault was a region 3 entry with FC=1
(large pud).

This allows to disambiguate all three possible scenarios:
* If ptep is not NULL, the fault was solved with a pte.
* If ptep is NULL and crste_region3 is 0, a segment entry with FC=1
  (large pmd) was used.
* If ptep is NULL and crste_region3 is 1, a region 3 entry with FC=1
  (large pud) was used.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/dat.h  | 1 +
 arch/s390/kvm/gmap.c | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/s390/kvm/dat.h b/arch/s390/kvm/dat.h
index 8f8278c44879..5d848e74e382 100644
--- a/arch/s390/kvm/dat.h
+++ b/arch/s390/kvm/dat.h
@@ -500,6 +500,7 @@ struct guest_fault {
 	bool write_attempt;	/* Write access attempted */
 	bool attempt_pfault;	/* Attempt a pfault first */
 	bool valid;		/* This entry contains valid data */
+	bool crste_region3;     /* Whether crstep refers to a region3 entry */
 	void (*callback)(struct guest_fault *f);
 	void *priv;
 };
diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 3c26e35af0ef..fe138d17caaf 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -531,6 +531,7 @@ static int gmap_handle_minor_crste_fault(struct gmap *gmap, struct guest_fault *
 	f->pfn = PHYS_PFN(large_crste_to_phys(oldcrste, f->gfn));
 	f->writable = oldcrste.s.fc1.w;
 
+	f->crste_region3 = is_pud(oldcrste);
 	/* Appropriate permissions already (race with another handler), nothing to do. */
 	if (!oldcrste.h.i && !(f->write_attempt && oldcrste.h.p))
 		return 0;
@@ -690,6 +691,7 @@ static int _gmap_link(struct kvm_s390_mmu_cache *mc, struct gmap *gmap, int leve
 			if (oldval.val != _CRSTE_EMPTY(oldval.h.tt).val &&
 			    crste_origin_large(oldval) != crste_origin_large(newval))
 				return -EAGAIN;
+			f->crste_region3 = is_pud(newval);
 		} while (!gmap_crstep_xchg_atomic(gmap, f->crstep, oldval, newval, f->gfn));
 		if (f->callback)
 			f->callback(f);
-- 
2.54.0

[PATCH v3 2/5] 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 | 45 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 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..f6521f16532a 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 +5737,50 @@ 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 first page of 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 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 errors happen while faulting-in the page (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)
+{
+	struct guest_fault f = { .gfn = gpa_to_gfn(range->gpa), };
+	gpa_t end;
+	int rc;
+
+	if (kvm_is_ucontrol(vcpu->kvm))
+		return -EINVAL;
+
+	rc = kvm_s390_faultin_gfn(vcpu, NULL, &f);
+	if (rc == PGM_ADDRESSING)
+		return -ENOENT;
+	if (rc > 0)
+		return -EIO;
+	if (rc < 0)
+		return rc;
+
+	if (f.ptep)
+		return PAGE_SIZE;
+
+	end = ALIGN(range->gpa + PAGE_SIZE, f.crste_region3 ? _REGION3_SIZE : HPAGE_SIZE);
+	return min(range->size, end - range->gpa);
+}
+
 /**
  * kvm_test_age_gfn() - test young
  * @kvm: the kvm instance
-- 
2.54.0

[PATCH v3 3/5] 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 v3 4/5] 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.

Remove the assertion  vcpu->run->exit_reason == KVM_EXIT_IO  since it
is x86-specific and redundant anyway.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/pre_fault_memory_test.c | 7 +------
 1 file changed, 1 insertion(+), 6 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..a0fcae3cb7a8 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;
 
@@ -193,11 +193,6 @@ 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,
-		    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
-		    run->exit_reason, exit_reason_str(run->exit_reason));
-
 	switch (get_ucall(vcpu, &uc)) {
 	case UCALL_ABORT:
 		REPORT_GUEST_ASSERT(uc);
-- 
2.54.0

[PATCH v3 5/5] 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 v3 0/5] KVM: s390: Implement KVM_PRE_FAULT_MEMORY

[Steffen Eiden]

On Wed, May 27, 2026 at 04:43:53PM +0200, Claudio Imbrenda wrote:
> Implement KVM_PRE_FAULT_MEMORY on s390, and update documentation and
> selftests accordingly.
> 

...

> Claudio Imbrenda (5):
>   KVM: s390: Track page size in struct guest_fault
>   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

LGTM. 

For the whole series:

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>