[PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

[PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

[Eric Farman]

Hi Hendrik, Janosch, et al,

Attached is a small series to allow ESA-mode guests in s390 VSIE.
It continues to expect that z/Architecture is the default,
unless opted in by userspace.

I look forward to your thoughts; thanks!

Eric Farman (3):
  KVM: s390: vsie: Allow non-zarch guests
  KVM: s390: vsie: Disable some bits when in ESA mode
  KVM: s390: vsie: Accommodate ESA prefix pages

Hendrik Brueckner (1):
  KVM: s390: Add KVM capability for ESA mode guests

 Documentation/virt/kvm/api.rst         |  8 +++++++
 arch/s390/include/asm/kvm_host.h       |  1 +
 arch/s390/include/asm/kvm_host_types.h |  3 +--
 arch/s390/kvm/kvm-s390.c               |  6 ++++++
 arch/s390/kvm/kvm-s390.h               |  5 ++++-
 arch/s390/kvm/vsie.c                   | 29 ++++++++++++++++++++++----
 include/uapi/linux/kvm.h               |  1 +
 7 files changed, 46 insertions(+), 7 deletions(-)

-- 
2.51.0

[PATCH v1 1/4] KVM: s390: vsie: Allow non-zarch guests

[Eric Farman]

Linux/KVM runs in z/Architecture-only mode. Although z/Architecture
is built upon a long history of hardware refinements, any other
CPU mode is not permitted.

Allow a userspace to explicitly enable the use of ESA mode for
nested guests, otherwise usage will be rejected.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
 arch/s390/include/asm/kvm_host.h | 1 +
 arch/s390/kvm/vsie.c             | 8 +++++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index 3039c88daa63..8a4f4a39f7a2 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -656,6 +656,7 @@ struct kvm_arch {
 	int user_stsi;
 	int user_instr0;
 	int user_operexec;
+	int allow_vsie_esamode;
 	struct s390_io_adapter *adapters[MAX_S390_IO_ADAPTERS];
 	wait_queue_head_t ipte_wq;
 	int ipte_lock_count;
diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index 72895dddc39a..888370a02ef7 100644
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -125,8 +125,9 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
 	int newflags, cpuflags = atomic_read(&scb_o->cpuflags);
 
-	/* we don't allow ESA/390 guests */
-	if (!(cpuflags & CPUSTAT_ZARCH))
+	newflags = 0;
+	/* we don't allow ESA/390 guests unless explicitly enabled */
+	if (!(cpuflags & CPUSTAT_ZARCH) && !vcpu->kvm->arch.allow_vsie_esamode)
 		return set_validity_icpt(scb_s, 0x0001U);
 
 	if (cpuflags & (CPUSTAT_RRF | CPUSTAT_MCDS))
@@ -135,7 +136,8 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 		return set_validity_icpt(scb_s, 0x0007U);
 
 	/* intervention requests will be set later */
-	newflags = CPUSTAT_ZARCH;
+	if (cpuflags & CPUSTAT_ZARCH)
+		newflags = CPUSTAT_ZARCH;
 	if (cpuflags & CPUSTAT_GED && test_kvm_facility(vcpu->kvm, 8))
 		newflags |= CPUSTAT_GED;
 	if (cpuflags & CPUSTAT_GED2 && test_kvm_facility(vcpu->kvm, 78)) {
-- 
2.51.0

[PATCH v1 2/4] KVM: s390: vsie: Disable some bits when in ESA mode

[Eric Farman]

In the event that a nested guest is put in ESA mode,
ensure that some bits are scrubbed from the shadow SCB.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
 arch/s390/kvm/vsie.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index 888370a02ef7..584fc7803632 100644
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -387,6 +387,17 @@ static int shadow_crycb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	return 0;
 }
 
+static void shadow_esa(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
+{
+	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
+
+	/* Ensure these bits are indeed turned off */
+	scb_s->eca &= ~ECA_VX;
+	scb_s->ecb &= ~ECB_GS;
+	scb_s->ecb3 &= ~ECB3_RI;
+	scb_s->ecd &= ~ECD_HOSTREGMGMT;
+}
+
 /* shadow (round up/down) the ibc to avoid validity icpt */
 static void prepare_ibc(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 {
@@ -590,6 +601,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	scb_s->hpid = HPID_VSIE;
 	scb_s->cpnc = scb_o->cpnc;
 
+	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_ZARCH))
+		shadow_esa(vcpu, vsie_page);
+
 	prepare_ibc(vcpu, vsie_page);
 	rc = shadow_crycb(vcpu, vsie_page);
 out:
-- 
2.51.0

[PATCH v1 3/4] KVM: s390: vsie: Accommodate ESA prefix pages

[Eric Farman]

The prefix page address occupies a different number
of bits for z/Architecture versus ESA mode. Adjust the
definition to cover both, and permit an ESA mode
address within the nested codepath.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
 arch/s390/include/asm/kvm_host_types.h | 3 +--
 arch/s390/kvm/kvm-s390.h               | 5 ++++-
 arch/s390/kvm/vsie.c                   | 7 ++++++-
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/kvm_host_types.h b/arch/s390/include/asm/kvm_host_types.h
index 1394d3fb648f..3f50942bdfe6 100644
--- a/arch/s390/include/asm/kvm_host_types.h
+++ b/arch/s390/include/asm/kvm_host_types.h
@@ -137,8 +137,7 @@ struct mcck_volatile_info {
 struct kvm_s390_sie_block {
 	atomic_t cpuflags;		/* 0x0000 */
 	__u32 : 1;			/* 0x0004 */
-	__u32 prefix : 18;
-	__u32 : 1;
+	__u32 prefix : 19;
 	__u32 ibc : 12;
 	__u8	reserved08[4];		/* 0x0008 */
 #define PROG_IN_SIE (1<<0)
diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
index bf1d7798c1af..c0ad8effe3d0 100644
--- a/arch/s390/kvm/kvm-s390.h
+++ b/arch/s390/kvm/kvm-s390.h
@@ -122,7 +122,9 @@ static inline int kvm_is_ucontrol(struct kvm *kvm)
 #endif
 }
 
-#define GUEST_PREFIX_SHIFT 13
+#define GUEST_PREFIX_SHIFT 12
+#define GUEST_PREFIX_ZARCH 0x7fffe
+#define GUEST_PREFIX_ESA   0x7ffff
 static inline u32 kvm_s390_get_prefix(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.sie_block->prefix << GUEST_PREFIX_SHIFT;
@@ -133,6 +135,7 @@ static inline void kvm_s390_set_prefix(struct kvm_vcpu *vcpu, u32 prefix)
 	VCPU_EVENT(vcpu, 3, "set prefix of cpu %03u to 0x%x", vcpu->vcpu_id,
 		   prefix);
 	vcpu->arch.sie_block->prefix = prefix >> GUEST_PREFIX_SHIFT;
+	vcpu->arch.sie_block->prefix &= GUEST_PREFIX_ZARCH;
 	kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
 	kvm_make_request(KVM_REQ_REFRESH_GUEST_PREFIX, vcpu);
 }
diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index 584fc7803632..e45378a01731 100644
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -479,7 +479,7 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
 	/* READ_ONCE does not work on bitfields - use a temporary variable */
 	const uint32_t __new_prefix = scb_o->prefix;
-	const uint32_t new_prefix = READ_ONCE(__new_prefix);
+	uint32_t new_prefix = READ_ONCE(__new_prefix);
 	const bool wants_tx = READ_ONCE(scb_o->ecb) & ECB_TE;
 	bool had_tx = scb_s->ecb & ECB_TE;
 	unsigned long new_mso = 0;
@@ -527,6 +527,11 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 
 	scb_s->icpua = scb_o->icpua;
 
+	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_ZARCH))
+		new_prefix &= GUEST_PREFIX_ESA;
+	else
+		new_prefix &= GUEST_PREFIX_ZARCH;
+
 	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_SM))
 		new_mso = READ_ONCE(scb_o->mso) & 0xfffffffffff00000UL;
 	/* if the hva of the prefix changes, we have to remap the prefix */
-- 
2.51.0

[PATCH v1 4/4] KVM: s390: Add KVM capability for ESA mode guests

[Eric Farman]

From: Hendrik Brueckner <brueckner@linux.ibm.com>

Now that all the bits are properly addressed, provide a mechanism
for testing ESA mode guests in nested configurations.

Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
[EF: Update commit message]
Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
 Documentation/virt/kvm/api.rst | 8 ++++++++
 arch/s390/kvm/kvm-s390.c       | 6 ++++++
 include/uapi/linux/kvm.h       | 1 +
 3 files changed, 15 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 032516783e96..feabfee0f714 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -9436,6 +9436,14 @@ KVM exits with the register state of either the L1 or L2 guest
 depending on which executed at the time of an exit. Userspace must
 take care to differentiate between these cases.
 
+8.47 KVM_CAP_S390_VSIE_ESAMODE
+------------------------------
+
+:Architectures: s390
+
+The presence of this capability indicates that the nested KVM guest can
+start in ESA mode.
+
 9. Known KVM API problems
 =========================
 
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index d7838334a338..3856af54b6fe 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -629,6 +629,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_IRQFD_RESAMPLE:
 	case KVM_CAP_S390_USER_OPEREXEC:
 	case KVM_CAP_S390_KEYOP:
+	case KVM_CAP_S390_VSIE_ESAMODE:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
@@ -926,6 +927,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
 		icpt_operexc_on_all_vcpus(kvm);
 		r = 0;
 		break;
+	case KVM_CAP_S390_VSIE_ESAMODE:
+		VM_EVENT(kvm, 3, "%s", "ENABLE: CAP_S390_VSIE_ESAMODE");
+		kvm->arch.allow_vsie_esamode = 1;
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 80364d4dbebb..9710184d883d 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -989,6 +989,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_ARM_SEA_TO_USER 245
 #define KVM_CAP_S390_USER_OPEREXEC 246
 #define KVM_CAP_S390_KEYOP 247
+#define KVM_CAP_S390_VSIE_ESAMODE 248
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
-- 
2.51.0

Re: [PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

[Christoph Hellwig]

On Wed, Apr 01, 2026 at 04:09:11AM +0200, Eric Farman wrote:
> Hi Hendrik, Janosch, et al,
> 
> Attached is a small series to allow ESA-mode guests in s390 VSIE.
> It continues to expect that z/Architecture is the default,
> unless opted in by userspace.
> 
> I look forward to your thoughts; thanks!

What is the use case for this?  Migrating existin VMs from 31-bit hosts
to 64-bit hosts?

Re: [PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

[Janosch Frank]

On 4/1/26 08:40, Christoph Hellwig wrote:
> On Wed, Apr 01, 2026 at 04:09:11AM +0200, Eric Farman wrote:
>> Hi Hendrik, Janosch, et al,
>>
>> Attached is a small series to allow ESA-mode guests in s390 VSIE.
>> It continues to expect that z/Architecture is the default,
>> unless opted in by userspace.
>>
>> I look forward to your thoughts; thanks!
> 
> What is the use case for this?  Migrating existin VMs from 31-bit hosts
> to 64-bit hosts?
> 

KVM is the latest hypervisor in a long list on s390. As such we 
implemented the newest architecture as our starting point and only 
architecture (zArch/64bit).

There are others which don't start their guests from zArch mode but 
start them in ESA and slowly move them through modes until they reach 
zArch. Those other hypervisors often implement emulation for features 
and devices that KVM doesn't support so there's some demand for using them.

Why do we nest those hypervisors under KVM? We have products that work 
with the KVM API so it's easier to nest than to re-write the product to 
use other hypervisors in the first place.

Re: [PATCH v1 1/4] KVM: s390: vsie: Allow non-zarch guests

[Janosch Frank]

On 4/1/26 04:09, Eric Farman wrote:
> Linux/KVM runs in z/Architecture-only mode. Although z/Architecture
> is built upon a long history of hardware refinements, any other
> CPU mode is not permitted.
> 
> Allow a userspace to explicitly enable the use of ESA mode for
> nested guests, otherwise usage will be rejected.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>   arch/s390/include/asm/kvm_host.h | 1 +
>   arch/s390/kvm/vsie.c             | 8 +++++---
>   2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 3039c88daa63..8a4f4a39f7a2 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -656,6 +656,7 @@ struct kvm_arch {
>   	int user_stsi;
>   	int user_instr0;
>   	int user_operexec;
> +	int allow_vsie_esamode;
>   	struct s390_io_adapter *adapters[MAX_S390_IO_ADAPTERS];
>   	wait_queue_head_t ipte_wq;
>   	int ipte_lock_count;
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 72895dddc39a..888370a02ef7 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -125,8 +125,9 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   	struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
>   	int newflags, cpuflags = atomic_read(&scb_o->cpuflags);
>   
> -	/* we don't allow ESA/390 guests */
> -	if (!(cpuflags & CPUSTAT_ZARCH))
> +	newflags = 0;

Not sure why you have that here but removed setting newflags below.

> +	/* we don't allow ESA/390 guests unless explicitly enabled */
> +	if (!(cpuflags & CPUSTAT_ZARCH) && !vcpu->kvm->arch.allow_vsie_esamode)
>   		return set_validity_icpt(scb_s, 0x0001U);
>   
>   	if (cpuflags & (CPUSTAT_RRF | CPUSTAT_MCDS))
> @@ -135,7 +136,8 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   		return set_validity_icpt(scb_s, 0x0007U);
>   
>   	/* intervention requests will be set later */
> -	newflags = CPUSTAT_ZARCH;

I'd leave this but set it to 0.

> +	if (cpuflags & CPUSTAT_ZARCH)
> +		newflags = CPUSTAT_ZARCH;
>   	if (cpuflags & CPUSTAT_GED && test_kvm_facility(vcpu->kvm, 8))
>   		newflags |= CPUSTAT_GED;
>   	if (cpuflags & CPUSTAT_GED2 && test_kvm_facility(vcpu->kvm, 78)) {

I can fix that up if you want.

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

Re: [PATCH v1 3/4] KVM: s390: vsie: Accommodate ESA prefix pages

[Janosch Frank]

On 4/1/26 04:09, Eric Farman wrote:
> The prefix page address occupies a different number
> of bits for z/Architecture versus ESA mode. Adjust the
> definition to cover both, and permit an ESA mode
> address within the nested codepath.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>   arch/s390/include/asm/kvm_host_types.h | 3 +--
>   arch/s390/kvm/kvm-s390.h               | 5 ++++-
>   arch/s390/kvm/vsie.c                   | 7 ++++++-
>   3 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host_types.h b/arch/s390/include/asm/kvm_host_types.h
> index 1394d3fb648f..3f50942bdfe6 100644
> --- a/arch/s390/include/asm/kvm_host_types.h
> +++ b/arch/s390/include/asm/kvm_host_types.h
> @@ -137,8 +137,7 @@ struct mcck_volatile_info {
>   struct kvm_s390_sie_block {
>   	atomic_t cpuflags;		/* 0x0000 */
>   	__u32 : 1;			/* 0x0004 */
> -	__u32 prefix : 18;
> -	__u32 : 1;
> +	__u32 prefix : 19;
>   	__u32 ibc : 12;
>   	__u8	reserved08[4];		/* 0x0008 */
>   #define PROG_IN_SIE (1<<0)
> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> index bf1d7798c1af..c0ad8effe3d0 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -122,7 +122,9 @@ static inline int kvm_is_ucontrol(struct kvm *kvm)
>   #endif
>   }
>   
> -#define GUEST_PREFIX_SHIFT 13
> +#define GUEST_PREFIX_SHIFT 12
> +#define GUEST_PREFIX_ZARCH 0x7fffe
> +#define GUEST_PREFIX_ESA   0x7ffff

Those should be GUEST_PREFIX_MASK_XXX.

GUEST_PREFIX_MASK_ZARCH can be used for set prefix and sigp set prefix 
emulation at a later time. Those functions currently use numeric constants.

The rest looks fine.

Re: [PATCH v1 4/4] KVM: s390: Add KVM capability for ESA mode guests

[Janosch Frank]

On 4/1/26 04:09, Eric Farman wrote:
> From: Hendrik Brueckner <brueckner@linux.ibm.com>
> 
> Now that all the bits are properly addressed, provide a mechanism
> for testing ESA mode guests in nested configurations.
> 
> Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
> [EF: Update commit message]

Should be:
[farman@linux.ibm.com: Updated commit message]

> Signed-off-by: Eric Farman <farman@linux.ibm.com>

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> ---
>   Documentation/virt/kvm/api.rst | 8 ++++++++
>   arch/s390/kvm/kvm-s390.c       | 6 ++++++
>   include/uapi/linux/kvm.h       | 1 +
>   3 files changed, 15 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 032516783e96..feabfee0f714 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -9436,6 +9436,14 @@ KVM exits with the register state of either the L1 or L2 guest
>   depending on which executed at the time of an exit. Userspace must
>   take care to differentiate between these cases.
>   
> +8.47 KVM_CAP_S390_VSIE_ESAMODE
> +------------------------------
> +
> +:Architectures: s390
> +
> +The presence of this capability indicates that the nested KVM guest can
> +start in ESA mode.
> +
>   9. Known KVM API problems
>   =========================
>   
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index d7838334a338..3856af54b6fe 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -629,6 +629,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	case KVM_CAP_IRQFD_RESAMPLE:
>   	case KVM_CAP_S390_USER_OPEREXEC:
>   	case KVM_CAP_S390_KEYOP:
> +	case KVM_CAP_S390_VSIE_ESAMODE:
>   		r = 1;
>   		break;
>   	case KVM_CAP_SET_GUEST_DEBUG2:
> @@ -926,6 +927,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
>   		icpt_operexc_on_all_vcpus(kvm);
>   		r = 0;
>   		break;
> +	case KVM_CAP_S390_VSIE_ESAMODE:
> +		VM_EVENT(kvm, 3, "%s", "ENABLE: CAP_S390_VSIE_ESAMODE");
> +		kvm->arch.allow_vsie_esamode = 1;
> +		r = 0;
> +		break;
>   	default:
>   		r = -EINVAL;
>   		break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 80364d4dbebb..9710184d883d 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -989,6 +989,7 @@ struct kvm_enable_cap {
>   #define KVM_CAP_ARM_SEA_TO_USER 245
>   #define KVM_CAP_S390_USER_OPEREXEC 246
>   #define KVM_CAP_S390_KEYOP 247
> +#define KVM_CAP_S390_VSIE_ESAMODE 248
>   
>   struct kvm_irq_routing_irqchip {
>   	__u32 irqchip;

Re: [PATCH v1 2/4] KVM: s390: vsie: Disable some bits when in ESA mode

[Christian Borntraeger]

Am 01.04.26 um 04:09 schrieb Eric Farman:
> In the event that a nested guest is put in ESA mode,
> ensure that some bits are scrubbed from the shadow SCB.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
apart from one thing below

> ---
>   arch/s390/kvm/vsie.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 888370a02ef7..584fc7803632 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -387,6 +387,17 @@ static int shadow_crycb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   	return 0;
>   }
>   
> +static void shadow_esa(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> +{
> +	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
> +
> +	/* Ensure these bits are indeed turned off */
> +	scb_s->eca &= ~ECA_VX;
> +	scb_s->ecb &= ~ECB_GS;
> +	scb_s->ecb3 &= ~ECB3_RI;
> +	scb_s->ecd &= ~ECD_HOSTREGMGMT;

shouldnt we also remove the TE bit (transactional execution)?

> +}
> +
>   /* shadow (round up/down) the ibc to avoid validity icpt */
>   static void prepare_ibc(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   {
> @@ -590,6 +601,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   	scb_s->hpid = HPID_VSIE;
>   	scb_s->cpnc = scb_o->cpnc;
>   
> +	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_ZARCH))
> +		shadow_esa(vcpu, vsie_page);
> +
>   	prepare_ibc(vcpu, vsie_page);
>   	rc = shadow_crycb(vcpu, vsie_page);
>   out:

Re: [PATCH v1 1/4] KVM: s390: vsie: Allow non-zarch guests

[Eric Farman]

On Wed, 2026-04-01 at 13:25 +0200, Janosch Frank wrote:
> On 4/1/26 04:09, Eric Farman wrote:
> > Linux/KVM runs in z/Architecture-only mode. Although z/Architecture
> > is built upon a long history of hardware refinements, any other
> > CPU mode is not permitted.
> > 
> > Allow a userspace to explicitly enable the use of ESA mode for
> > nested guests, otherwise usage will be rejected.
> > 
> > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > ---
> >   arch/s390/include/asm/kvm_host.h | 1 +
> >   arch/s390/kvm/vsie.c             | 8 +++++---
> >   2 files changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> > index 3039c88daa63..8a4f4a39f7a2 100644
> > --- a/arch/s390/include/asm/kvm_host.h
> > +++ b/arch/s390/include/asm/kvm_host.h
> > @@ -656,6 +656,7 @@ struct kvm_arch {
> >   	int user_stsi;
> >   	int user_instr0;
> >   	int user_operexec;
> > +	int allow_vsie_esamode;
> >   	struct s390_io_adapter *adapters[MAX_S390_IO_ADAPTERS];
> >   	wait_queue_head_t ipte_wq;
> >   	int ipte_lock_count;
> > diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> > index 72895dddc39a..888370a02ef7 100644
> > --- a/arch/s390/kvm/vsie.c
> > +++ b/arch/s390/kvm/vsie.c
> > @@ -125,8 +125,9 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> >   	struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
> >   	int newflags, cpuflags = atomic_read(&scb_o->cpuflags);
> >   
> > -	/* we don't allow ESA/390 guests */
> > -	if (!(cpuflags & CPUSTAT_ZARCH))
> > +	newflags = 0;
> 
> Not sure why you have that here but removed setting newflags below.

Yeah, I don't have any good reason for it here.

> 
> > +	/* we don't allow ESA/390 guests unless explicitly enabled */
> > +	if (!(cpuflags & CPUSTAT_ZARCH) && !vcpu->kvm->arch.allow_vsie_esamode)
> >   		return set_validity_icpt(scb_s, 0x0001U);
> >   
> >   	if (cpuflags & (CPUSTAT_RRF | CPUSTAT_MCDS))
> > @@ -135,7 +136,8 @@ static int prepare_cpuflags(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> >   		return set_validity_icpt(scb_s, 0x0007U);
> >   
> >   	/* intervention requests will be set later */
> > -	newflags = CPUSTAT_ZARCH;
> 
> I'd leave this but set it to 0.
> 
> > +	if (cpuflags & CPUSTAT_ZARCH)
> > +		newflags = CPUSTAT_ZARCH;
> >   	if (cpuflags & CPUSTAT_GED && test_kvm_facility(vcpu->kvm, 8))
> >   		newflags |= CPUSTAT_GED;
> >   	if (cpuflags & CPUSTAT_GED2 && test_kvm_facility(vcpu->kvm, 78)) {
> 
> I can fix that up if you want.

This makes more sense. And if you don't mind, that would be wonderful!

> 
> Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

Thank you!

Re: [PATCH v1 3/4] KVM: s390: vsie: Accommodate ESA prefix pages

[Eric Farman]

On Wed, 2026-04-01 at 13:45 +0200, Janosch Frank wrote:
> On 4/1/26 04:09, Eric Farman wrote:
> > The prefix page address occupies a different number
> > of bits for z/Architecture versus ESA mode. Adjust the
> > definition to cover both, and permit an ESA mode
> > address within the nested codepath.
> > 
> > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > ---
> >   arch/s390/include/asm/kvm_host_types.h | 3 +--
> >   arch/s390/kvm/kvm-s390.h               | 5 ++++-
> >   arch/s390/kvm/vsie.c                   | 7 ++++++-
> >   3 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/kvm_host_types.h b/arch/s390/include/asm/kvm_host_types.h
> > index 1394d3fb648f..3f50942bdfe6 100644
> > --- a/arch/s390/include/asm/kvm_host_types.h
> > +++ b/arch/s390/include/asm/kvm_host_types.h
> > @@ -137,8 +137,7 @@ struct mcck_volatile_info {
> >   struct kvm_s390_sie_block {
> >   	atomic_t cpuflags;		/* 0x0000 */
> >   	__u32 : 1;			/* 0x0004 */
> > -	__u32 prefix : 18;
> > -	__u32 : 1;
> > +	__u32 prefix : 19;
> >   	__u32 ibc : 12;
> >   	__u8	reserved08[4];		/* 0x0008 */
> >   #define PROG_IN_SIE (1<<0)
> > diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> > index bf1d7798c1af..c0ad8effe3d0 100644
> > --- a/arch/s390/kvm/kvm-s390.h
> > +++ b/arch/s390/kvm/kvm-s390.h
> > @@ -122,7 +122,9 @@ static inline int kvm_is_ucontrol(struct kvm *kvm)
> >   #endif
> >   }
> >   
> > -#define GUEST_PREFIX_SHIFT 13
> > +#define GUEST_PREFIX_SHIFT 12
> > +#define GUEST_PREFIX_ZARCH 0x7fffe
> > +#define GUEST_PREFIX_ESA   0x7ffff
> 
> Those should be GUEST_PREFIX_MASK_XXX.

Oops, I even thought it would be better with a MASK in there; just forgot to add it.

> 
> GUEST_PREFIX_MASK_ZARCH can be used for set prefix and sigp set prefix 
> emulation at a later time. Those functions currently use numeric constants.

Sure. I'll add that to the followup list.

> 
> The rest looks fine.

Re: [PATCH v1 2/4] KVM: s390: vsie: Disable some bits when in ESA mode

[Eric Farman]

On Wed, 2026-04-01 at 14:58 +0200, Christian Borntraeger wrote:
> 
> Am 01.04.26 um 04:09 schrieb Eric Farman:
> > In the event that a nested guest is put in ESA mode,
> > ensure that some bits are scrubbed from the shadow SCB.
> > 
> > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> 
> Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> apart from one thing below
> 
> > ---
> >   arch/s390/kvm/vsie.c | 14 ++++++++++++++
> >   1 file changed, 14 insertions(+)
> > 
> > diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> > index 888370a02ef7..584fc7803632 100644
> > --- a/arch/s390/kvm/vsie.c
> > +++ b/arch/s390/kvm/vsie.c
> > @@ -387,6 +387,17 @@ static int shadow_crycb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> >   	return 0;
> >   }
> >   
> > +static void shadow_esa(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> > +{
> > +	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
> > +
> > +	/* Ensure these bits are indeed turned off */
> > +	scb_s->eca &= ~ECA_VX;
> > +	scb_s->ecb &= ~ECB_GS;
> > +	scb_s->ecb3 &= ~ECB3_RI;
> > +	scb_s->ecd &= ~ECD_HOSTREGMGMT;
> 
> shouldnt we also remove the TE bit (transactional execution)?

Ahh, good point. Yeah, that should be removed.

@Janosch, I'll spin a quick v2 with this and the other nits, so you don't have to do the fixups.
Just want to doublecheck this one.

> 
> > +}
> > +
> >   /* shadow (round up/down) the ibc to avoid validity icpt */
> >   static void prepare_ibc(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> >   {
> > @@ -590,6 +601,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> >   	scb_s->hpid = HPID_VSIE;
> >   	scb_s->cpnc = scb_o->cpnc;
> >   
> > +	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_ZARCH))
> > +		shadow_esa(vcpu, vsie_page);
> > +
> >   	prepare_ibc(vcpu, vsie_page);
> >   	rc = shadow_crycb(vcpu, vsie_page);
> >   out:

Re: [PATCH v1 2/4] KVM: s390: vsie: Disable some bits when in ESA mode

[Janosch Frank]

On 4/1/26 15:46, Eric Farman wrote:
> On Wed, 2026-04-01 at 14:58 +0200, Christian Borntraeger wrote:
>>
>> Am 01.04.26 um 04:09 schrieb Eric Farman:
>>> In the event that a nested guest is put in ESA mode,
>>> ensure that some bits are scrubbed from the shadow SCB.
>>>
>>> Signed-off-by: Eric Farman <farman@linux.ibm.com>
>>
>> Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
>> apart from one thing below
>>
>>> ---
>>>    arch/s390/kvm/vsie.c | 14 ++++++++++++++
>>>    1 file changed, 14 insertions(+)
>>>
>>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>>> index 888370a02ef7..584fc7803632 100644
>>> --- a/arch/s390/kvm/vsie.c
>>> +++ b/arch/s390/kvm/vsie.c
>>> @@ -387,6 +387,17 @@ static int shadow_crycb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>    	return 0;
>>>    }
>>>    
>>> +static void shadow_esa(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>> +{
>>> +	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
>>> +
>>> +	/* Ensure these bits are indeed turned off */
>>> +	scb_s->eca &= ~ECA_VX;
>>> +	scb_s->ecb &= ~ECB_GS;
>>> +	scb_s->ecb3 &= ~ECB3_RI;
>>> +	scb_s->ecd &= ~ECD_HOSTREGMGMT;
>>
>> shouldnt we also remove the TE bit (transactional execution)?
> 
> Ahh, good point. Yeah, that should be removed.
> 
> @Janosch, I'll spin a quick v2 with this and the other nits, so you don't have to do the fixups.
> Just want to doublecheck this one.

Great, thanks

> 
>>
>>> +}
>>> +
>>>    /* shadow (round up/down) the ibc to avoid validity icpt */
>>>    static void prepare_ibc(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>    {
>>> @@ -590,6 +601,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>>    	scb_s->hpid = HPID_VSIE;
>>>    	scb_s->cpnc = scb_o->cpnc;
>>>    
>>> +	if (!(atomic_read(&scb_s->cpuflags) & CPUSTAT_ZARCH))
>>> +		shadow_esa(vcpu, vsie_page);
>>> +
>>>    	prepare_ibc(vcpu, vsie_page);
>>>    	rc = shadow_crycb(vcpu, vsie_page);
>>>    out:

Re: [PATCH v1 0/4] KVM: s390: Permit ESA mode guests in VSIE

[Christoph Hellwig]

On Wed, Apr 01, 2026 at 12:00:59PM +0200, Janosch Frank wrote:
> KVM is the latest hypervisor in a long list on s390. As such we implemented
> the newest architecture as our starting point and only architecture
> (zArch/64bit).
> 
> There are others which don't start their guests from zArch mode but start
> them in ESA and slowly move them through modes until they reach zArch. Those
> other hypervisors often implement emulation for features and devices that
> KVM doesn't support so there's some demand for using them.
> 
> Why do we nest those hypervisors under KVM? We have products that work with
> the KVM API so it's easier to nest than to re-write the product to use other
> hypervisors in the first place.

Makes sense.  Maybe mention this in the cover letter, and also that
you're not looking into full device model support for older systems?