[RESEND PATCH 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[RESEND PATCH 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[Amit Machhiwal]

This series addresses a few bug fixes across KVM PPC Book3S HV
locking/synchronization and paired-single instruction decoding:

Patches 1 & 2 fix synchronization and preemption issues introduced in
commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for
HPT hcalls and page faults"):

- Patch 1 adds SRCU read lock protection when walking memslots during
  virtual-mode HPT hcalls to prevent use-after-free races with concurrent
  memslot updates.

- Patch 2 adds preempt_disable() around virtual-mode HPTE bit-lock holders
  and raw spinlock usage to prevent CPU stalls and deadlocks on preemption.

Patch 3 fixes paired-single D-form instruction emulation:

- Patch 3 fixes get_d_signext() to correctly extract the full 12-bit D
  displacement field and perform proper two's-complement sign extension.

Testing:
========
All test kernel builds were compiled with CONFIG_DEBUG_ATOMIC_SLEEP=y.
The following scenarios were verified:

1. Power9 PowerNV (L0) in Radix mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guests in both Radix and Hash modes.
   - Ran kernel build workload inside guests — no errors observed.
   - Booted L1 guest with the same kernel — booted and ran cleanly.

2. Power9 PowerNV (L0) in Hash mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guest in Hash mode.
   - Ran kernel build workload inside guest — no errors observed.
   - Booted L1 nested guest with the same kernel — booted and ran cleanly.

3. Power10 LPAR (L1):
   - Booted Power10 LPAR with the patched kernel.
   - Booted KVM guest and ran workloads — no errors observed.

Amit Machhiwal (3):
  KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls
  KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE
    bit-lock users
  KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single
    D-form

 arch/powerpc/kvm/book3s_hv.c             | 76 +++++++++++++-----------
 arch/powerpc/kvm/book3s_paired_singles.c |  7 +--
 2 files changed, 42 insertions(+), 41 deletions(-)


base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
-- 
2.54.0 (Apple Git-157)

[RESEND PATCH 1/3] KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls

[Amit Machhiwal]

The virtual-mode HPT hcall handlers (H_ENTER, H_REMOVE, H_BULK_REMOVE,
H_CLEAR_REF, H_CLEAR_MOD) call into book3s_hv_rm_mmu.c, which accesses
memslots via kvm_memslots_raw().  kvm_memslots_raw() uses
rcu_dereference_raw_check() to bypass SRCU lockdep annotation checking.
This is safe in real mode because the entire guest entry/exit is wrapped
in srcu_read_lock/unlock inside kvmppc_run_core() and
kvmhv_run_single_vcpu().

However, since commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual
mode handlers for HPT hcalls and page faults"), these same handlers are
also executed in virtual mode via kvmppc_pseries_do_hcall(), which runs
after SRCU has already been released on guest exit.

A concurrent KVM_SET_USER_MEMORY_REGION deletion or move can therefore
call synchronize_srcu_expedited() — which does not wait for this thread —
and then kfree(slot) and vfree(slot->arch.rmap) while the hcall handler
still holds a raw pointer to the memslot.  lock_rmap() then writes to
freed vmalloc memory, leading to use-after-free and memory corruption.
Because kvm_memslots_raw() suppresses lockdep checks, this race is
entirely silent.

Fix this by factoring out the 7 HPT hcall handlers (H_REMOVE, H_ENTER,
H_READ, H_CLEAR_MOD, H_CLEAR_REF, H_PROTECT, H_BULK_REMOVE) into a
helper function, kvmppc_pseries_do_hpt_hcall(), and wrapping its call
site in srcu_read_lock(&kvm->srcu) / srcu_read_unlock(&kvm->srcu, idx).
Targeting only the HPT hcalls avoids wrapping non-HPT hcalls that sleep
(such as H_CONFER, H_REGISTER_VPA, H_PAGE_INIT) or handlers that already
acquire SRCU internally (such as H_RTAS).

Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
Cc: stable@vger.kernel.org # v5.14+
Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv.c | 70 ++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 35 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index dbac3573b2c8..aa51968e206a 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1159,6 +1159,38 @@ static long kvmppc_h_rpt_invalidate(struct kvm_vcpu *vcpu,
 	return H_SUCCESS;
 }
 
+static long kvmppc_pseries_do_hpt_hcall(struct kvm_vcpu *vcpu, unsigned long req)
+{
+	switch (req) {
+	case H_REMOVE:
+		return kvmppc_h_remove(vcpu, kvmppc_get_gpr(vcpu, 4),
+				       kvmppc_get_gpr(vcpu, 5),
+				       kvmppc_get_gpr(vcpu, 6));
+	case H_ENTER:
+		return kvmppc_h_enter(vcpu, kvmppc_get_gpr(vcpu, 4),
+				      kvmppc_get_gpr(vcpu, 5),
+				      kvmppc_get_gpr(vcpu, 6),
+				      kvmppc_get_gpr(vcpu, 7));
+	case H_READ:
+		return kvmppc_h_read(vcpu, kvmppc_get_gpr(vcpu, 4),
+				     kvmppc_get_gpr(vcpu, 5));
+	case H_CLEAR_MOD:
+		return kvmppc_h_clear_mod(vcpu, kvmppc_get_gpr(vcpu, 4),
+					  kvmppc_get_gpr(vcpu, 5));
+	case H_CLEAR_REF:
+		return kvmppc_h_clear_ref(vcpu, kvmppc_get_gpr(vcpu, 4),
+					  kvmppc_get_gpr(vcpu, 5));
+	case H_PROTECT:
+		return kvmppc_h_protect(vcpu, kvmppc_get_gpr(vcpu, 4),
+					kvmppc_get_gpr(vcpu, 5),
+					kvmppc_get_gpr(vcpu, 6));
+	case H_BULK_REMOVE:
+		return kvmppc_h_bulk_remove(vcpu);
+	}
+
+	return H_FUNCTION;
+}
+
 int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
@@ -1174,47 +1206,15 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 
 	switch (req) {
 	case H_REMOVE:
-		ret = kvmppc_h_remove(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_ENTER:
-		ret = kvmppc_h_enter(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6),
-					kvmppc_get_gpr(vcpu, 7));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_READ:
-		ret = kvmppc_h_read(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_CLEAR_MOD:
-		ret = kvmppc_h_clear_mod(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_CLEAR_REF:
-		ret = kvmppc_h_clear_ref(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_PROTECT:
-		ret = kvmppc_h_protect(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_BULK_REMOVE:
-		ret = kvmppc_h_bulk_remove(vcpu);
+		idx = srcu_read_lock(&kvm->srcu);
+		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
+		srcu_read_unlock(&kvm->srcu, idx);
 		if (ret == H_TOO_HARD)
 			return RESUME_HOST;
 		break;
-- 
2.54.0 (Apple Git-157)

Re: [RESEND PATCH 1/3] KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls

[Ritesh Harjani]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> The virtual-mode HPT hcall handlers (H_ENTER, H_REMOVE, H_BULK_REMOVE,
> H_CLEAR_REF, H_CLEAR_MOD) call into book3s_hv_rm_mmu.c, which accesses
> memslots via kvm_memslots_raw().  kvm_memslots_raw() uses
> rcu_dereference_raw_check() to bypass SRCU lockdep annotation checking.
> This is safe in real mode because the entire guest entry/exit is wrapped
> in srcu_read_lock/unlock inside kvmppc_run_core() and
> kvmhv_run_single_vcpu().
>
> However, since commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual
> mode handlers for HPT hcalls and page faults"), these same handlers are
> also executed in virtual mode via kvmppc_pseries_do_hcall(), which runs
> after SRCU has already been released on guest exit.
>
> A concurrent KVM_SET_USER_MEMORY_REGION deletion or move can therefore
> call synchronize_srcu_expedited() — which does not wait for this thread —
> and then kfree(slot) and vfree(slot->arch.rmap) while the hcall handler
> still holds a raw pointer to the memslot.  lock_rmap() then writes to
> freed vmalloc memory, leading to use-after-free and memory corruption.
> Because kvm_memslots_raw() suppresses lockdep checks, this race is
> entirely silent.
>
> Fix this by factoring out the 7 HPT hcall handlers (H_REMOVE, H_ENTER,
> H_READ, H_CLEAR_MOD, H_CLEAR_REF, H_PROTECT, H_BULK_REMOVE) into a
> helper function, kvmppc_pseries_do_hpt_hcall(), and wrapping its call
> site in srcu_read_lock(&kvm->srcu) / srcu_read_unlock(&kvm->srcu, idx).
> Targeting only the HPT hcalls avoids wrapping non-HPT hcalls that sleep
> (such as H_CONFER, H_REGISTER_VPA, H_PAGE_INIT) or handlers that already
> acquire SRCU internally (such as H_RTAS).
>
> Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
> Cc: stable@vger.kernel.org # v5.14+
> Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 70 ++++++++++++++++++------------------
>  1 file changed, 35 insertions(+), 35 deletions(-)

I had looked at this path in depth earlier itself. LGTM.
Please feel free to add:

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

[RESEND PATCH 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[Amit Machhiwal]

kvmppc_hv_find_lock_hpte() requires virtual-mode callers to run with
preemption disabled, because it can return with HPTE_V_HVLOCK still held
until the caller later unlocks the HPTE.  Existing virtual-mode callers
in book3s_64_mmu_hv.c already follow that rule.

Two virtual-mode call paths do not:

1. kvmppc_handle_exit_hv() calls kvmppc_hpte_hv_fault() for hash-mode
   data-side and instruction-side faults after guest exit, with
   preemption already enabled.  kvmppc_hpte_hv_fault() calls
   kvmppc_hv_find_lock_hpte() directly.

2. kvmppc_pseries_do_hcall() executes virtual-mode handlers for HPT
   hcalls via kvmppc_pseries_do_hpt_hcall() with preemption enabled.
   The handlers for H_ENTER, H_REMOVE, H_READ, H_CLEAR_MOD, H_CLEAR_REF,
   H_PROTECT, and H_BULK_REMOVE spin on try_lock_hpte() or lock_rmap(),
   and H_ENTER also reaches kvmppc_do_h_enter(), which uses
   arch_spin_lock() on kvm->mmu_lock.  That raw lock choice is
   intentional because kvmppc_do_h_enter() is also used by real-mode
   callers, so the correct fix is to establish the proper preemption
   context at the virtual-mode caller boundary.

If a vCPU thread is preempted while holding HPTE_V_HVLOCK, any other
thread on the same CPU trying to acquire the same bit-lock can spin
indefinitely.  The same applies to lock_rmap() users and to the H_ENTER
path while holding the raw mmu_lock.

Fix this by adding preempt_disable()/preempt_enable() pairs around the
two kvmppc_hpte_hv_fault() call sites in kvmppc_handle_exit_hv(), and
around the kvmppc_pseries_do_hpt_hcall() invocation in
kvmppc_pseries_do_hcall().  Targeting only the HPT hcall helper avoids
wrapping non-HPT hcalls (such as H_RTAS, H_CONFER, H_REGISTER_VPA) that
perform page faults, blocking memory allocations, or context switches.

Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index aa51968e206a..52f72f30baf0 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1212,9 +1212,11 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 	case H_CLEAR_REF:
 	case H_PROTECT:
 	case H_BULK_REMOVE:
+		preempt_disable();
 		idx = srcu_read_lock(&kvm->srcu);
 		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
 		srcu_read_unlock(&kvm->srcu, idx);
+		preempt_enable();
 		if (ret == H_TOO_HARD)
 			return RESUME_HOST;
 		break;
@@ -1834,8 +1836,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 		else
 			vsid = vcpu->arch.fault_gpa;
 
+		preempt_disable();
 		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
 				vsid, vcpu->arch.fault_dsisr, true);
+		preempt_enable();
 		if (err == 0) {
 			r = RESUME_GUEST;
 		} else if (err == -1 || err == -2) {
@@ -1881,8 +1885,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 		else
 			vsid = vcpu->arch.fault_gpa;
 
+		preempt_disable();
 		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
 				vsid, vcpu->arch.fault_dsisr, false);
+		preempt_enable();
 		if (err == 0) {
 			r = RESUME_GUEST;
 		} else if (err == -1) {
-- 
2.54.0 (Apple Git-157)

Re: [RESEND PATCH 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[Ritesh Harjani]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> kvmppc_hv_find_lock_hpte() requires virtual-mode callers to run with
> preemption disabled, because it can return with HPTE_V_HVLOCK still held
> until the caller later unlocks the HPTE.  Existing virtual-mode callers
> in book3s_64_mmu_hv.c already follow that rule.
>

That make sense. LGTM. Please feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Re: [RESEND PATCH 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[Shrikanth Hegde]

On 9/28/26 5:58 PM, Amit Machhiwal wrote:
> kvmppc_hv_find_lock_hpte() requires virtual-mode callers to run with
> preemption disabled, because it can return with HPTE_V_HVLOCK still held
> until the caller later unlocks the HPTE.  Existing virtual-mode callers
> in book3s_64_mmu_hv.c already follow that rule.
> 
> Two virtual-mode call paths do not:
> 
> 1. kvmppc_handle_exit_hv() calls kvmppc_hpte_hv_fault() for hash-mode
>     data-side and instruction-side faults after guest exit, with
>     preemption already enabled.  kvmppc_hpte_hv_fault() calls
>     kvmppc_hv_find_lock_hpte() directly.
> 
> 2. kvmppc_pseries_do_hcall() executes virtual-mode handlers for HPT
>     hcalls via kvmppc_pseries_do_hpt_hcall() with preemption enabled.
>     The handlers for H_ENTER, H_REMOVE, H_READ, H_CLEAR_MOD, H_CLEAR_REF,
>     H_PROTECT, and H_BULK_REMOVE spin on try_lock_hpte() or lock_rmap(),
>     and H_ENTER also reaches kvmppc_do_h_enter(), which uses
>     arch_spin_lock() on kvm->mmu_lock.  That raw lock choice is
>     intentional because kvmppc_do_h_enter() is also used by real-mode
>     callers, so the correct fix is to establish the proper preemption
>     context at the virtual-mode caller boundary.
> 
> If a vCPU thread is preempted while holding HPTE_V_HVLOCK, any other
> thread on the same CPU trying to acquire the same bit-lock can spin
> indefinitely.  The same applies to lock_rmap() users and to the H_ENTER
> path while holding the raw mmu_lock.
> 
> Fix this by adding preempt_disable()/preempt_enable() pairs around the
> two kvmppc_hpte_hv_fault() call sites in kvmppc_handle_exit_hv(), and
> around the kvmppc_pseries_do_hpt_hcall() invocation in
> kvmppc_pseries_do_hcall().  Targeting only the HPT hcall helper avoids
> wrapping non-HPT hcalls (such as H_RTAS, H_CONFER, H_REGISTER_VPA) that
> perform page faults, blocking memory allocations, or context switches.
> 
> Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
> Cc: stable@vger.kernel.org # v5.14+
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>   arch/powerpc/kvm/book3s_hv.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index aa51968e206a..52f72f30baf0 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1212,9 +1212,11 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
>   	case H_CLEAR_REF:
>   	case H_PROTECT:
>   	case H_BULK_REMOVE:
> +		preempt_disable();
>   		idx = srcu_read_lock(&kvm->srcu);

Please add a comment above kvmppc_pseries_do_hpt_hcall, saying preemption
is disabled and one cannot make any blocking call.

>   		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
>   		srcu_read_unlock(&kvm->srcu, idx);
> +		preempt_enable();
>   		if (ret == H_TOO_HARD)
>   			return RESUME_HOST;
>   		break;
> @@ -1834,8 +1836,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
>   		else
>   			vsid = vcpu->arch.fault_gpa;
>   
> +		preempt_disable();
>   		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
>   				vsid, vcpu->arch.fault_dsisr, true);
> +		preempt_enable();
>   		if (err == 0) {
>   			r = RESUME_GUEST;
>   		} else if (err == -1 || err == -2) {
> @@ -1881,8 +1885,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
>   		else
>   			vsid = vcpu->arch.fault_gpa;
>   
> +		preempt_disable();
>   		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
>   				vsid, vcpu->arch.fault_dsisr, false);
> +		preempt_enable();
>   		if (err == 0) {
>   			r = RESUME_GUEST;
>   		} else if (err == -1) {

Please check if below is possible case.

https://sashiko.dev/#/patchset/20260928113704.48912-4-amachhiw%40linux.ibm.com

Re: [RESEND PATCH 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[Amit Machhiwal]

Hi Shrikanth,

Thanks for taking a look at this patch.  Please find my responses inline.

On 2026/09/29 11:48 AM, Shrikanth Hegde wrote:
> 
> 
> On 9/28/26 5:58 PM, Amit Machhiwal wrote:
> > kvmppc_hv_find_lock_hpte() requires virtual-mode callers to run with
> > preemption disabled, because it can return with HPTE_V_HVLOCK still held
> > until the caller later unlocks the HPTE.  Existing virtual-mode callers
> > in book3s_64_mmu_hv.c already follow that rule.
> > 

<..snip..>

> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index aa51968e206a..52f72f30baf0 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -1212,9 +1212,11 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
> >   	case H_CLEAR_REF:
> >   	case H_PROTECT:
> >   	case H_BULK_REMOVE:
> > +		preempt_disable();
> >   		idx = srcu_read_lock(&kvm->srcu);
> 
> Please add a comment above kvmppc_pseries_do_hpt_hcall, saying preemption
> is disabled and one cannot make any blocking call.

Sure, will do.

> 
> >   		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
> >   		srcu_read_unlock(&kvm->srcu, idx);
> > +		preempt_enable();
> >   		if (ret == H_TOO_HARD)
> >   			return RESUME_HOST;
> >   		break;
> > @@ -1834,8 +1836,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
> >   		else
> >   			vsid = vcpu->arch.fault_gpa;
> > +		preempt_disable();
> >   		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
> >   				vsid, vcpu->arch.fault_dsisr, true);
> > +		preempt_enable();
> >   		if (err == 0) {
> >   			r = RESUME_GUEST;
> >   		} else if (err == -1 || err == -2) {
> > @@ -1881,8 +1885,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
> >   		else
> >   			vsid = vcpu->arch.fault_gpa;
> > +		preempt_disable();
> >   		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
> >   				vsid, vcpu->arch.fault_dsisr, false);
> > +		preempt_enable();
> >   		if (err == 0) {
> >   			r = RESUME_GUEST;
> >   		} else if (err == -1) {
> 
> Please check if below is possible case.
> 
> https://sashiko.dev/#/patchset/20260928113704.48912-4-amachhiw%40linux.ibm.com

Thank you for raising this point and bringing Sashiko AI's review comment to
attention.  After conducting a deep-dive audit of the preemption paths and the
virtual-mode locking context, it looks like Sashiko's finding is legitimate, and
extremely subtle.

The Deadlock Mechanism:
=======================

In this patch we added preempt_disable() / preempt_enable() pairs exclusively
around the guest vCPU HPT hcall paths (kvmppc_pseries_do_hpt_hcall) and page
fault paths (kvmppc_hpte_hv_fault) in virtual mode to satisfy the HPTE locking
preemption contract.

However, several host-side virtual-mode paths still acquire and hold the HPTE
bit-lock (HPTE_V_HVLOCK) with preemption enabled.  Specifically:

- While MMU notifier paths (unmapping, aging) and the dirty-log path are safe
  (since they are called with kvm->mmu_lock held, which disables preemption),
  the memslot flushing path (kvmppc_core_flush_memslot_hv() ->
  kvm_unmap_rmapp()) runs with only the slots_arch_lock mutex held, leaving
  preemption fully enabled.

If a host-side thread executing kvm_unmap_rmapp() successfully acquires the
HPTE_V_HVLOCK on CPU X and is preempted, a guest vCPU thread subsequently
scheduled on CPU X will enter the hcall or fault path, disable preemption (via
the current change), and spin indefinitely on try_lock_hpte().  Because
preemption is disabled on the spinning vCPU, it will never yield CPU X.  The
preempted host-side lock owner can never be rescheduled on CPU X to release the
lock.

This converts a potential latency/livelock issue into a permanent hard deadlock.

Proposed Solution:
==================

To make this fix completely watertight and eliminate the deadlock window, no
thread must ever be preempted while holding HPTE_V_HVLOCK in virtual mode.  This
means we must disable preemption during the lock-hold window on both the
vCPU-side and the host-side paths.

In the next version, I'll be making following changes:

Host-Side Preemption Disabling: Add preempt_disable() / preempt_enable()
brackets around the HPTE_V_HVLOCK hold windows inside the four host-side
virtual-mode functions in arch/powerpc/kvm/book3s_64_mmu_hv.c:

  - kvm_unmap_rmapp()
  - kvm_age_rmapp()
  - kvm_test_clear_dirty_npages()
  - resize_hpt_rehash_hpte()

For example, inside kvm_unmap_rmapp():

 diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
 index 2ccb3d138f46..59da958e09cb 100644
 --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
 +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
 @@ -823,7 +823,9 @@ static void kvm_unmap_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
                  */
                 i = *rmapp & KVMPPC_RMAP_INDEX;
                 hptep = (__be64 *) (kvm->arch.hpt.virt + (i << 4));
 +               preempt_disable();
                 if (!try_lock_hpte(hptep, HPTE_V_HVLOCK)) {
 +                       preempt_enable();
                         /* unlock rmap before spinning on the HPTE lock */
                         unlock_rmap(rmapp);
                         while (be64_to_cpu(hptep[0]) & HPTE_V_HVLOCK)
 @@ -834,6 +836,7 @@ static void kvm_unmap_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
                 kvmppc_unmap_hpte(kvm, i, memslot, rmapp, gfn);
                 unlock_rmap(rmapp);
                 __unlock_hpte(hptep, be64_to_cpu(hptep[0]));
 +               preempt_enable();
         }
  }

To ensure complete consistency and verify there are no other potential
preemption deadlock sites, I conducted a full audit of all 8 occurrences of
try_lock_hpte() inside arch/powerpc/kvm/book3s_64_mmu_hv.c.

The proposed v2 fixes simply bring the host-side lock users into alignment with
the established, pre-existing design pattern used by every other virtual-mode
caller of try_lock_hpte() in this file:

- Established safe callers (4) — already wrapped in preempt_disable() /
  preempt_enable() blocks by design:
	- kvmppc_book3s_hv_page_fault()
	- read_hpte_for_cma()
	- kvmppc_hpte_hv_page_debug()

- Symmetrical alignment in v2 (4):
	- kvm_unmap_rmapp()
	- kvm_age_rmapp()
	- kvm_test_clear_dirty_npages()
	- resize_hpt_rehash_hpte()

This ensures 100% preemption-safety symmetry across all virtual-mode
lock-holding windows in KVM.

I hope this sounds good to you.

Thanks,
Amit

[RESEND PATCH 3/3] KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single D-form

[Amit Machhiwal]

get_d_signext() has two compounding bugs since its introduction in 2010:

1. The extraction mask 0x8FF silently drops bits 8-10 of the 12-bit D
   field (PPC ISA bits 21-23), corrupting any displacement that has any
   of those bits set.

2. The sign-magnitude idiom "-(d & 0x7ff)" is wrong for two's-complement:
   for D=0xFFC (encoding of -4) it returns -252 instead of -4.

Together these errors produce a wrong effective address for psq_l, psq_lu,
psq_st, and psq_stu whenever the displacement is negative or is a positive
value >= 0x100 with bits 8-10 set — essentially any real-world paired-
single stack-relative access.

The D field occupies the bottom 12 bits of the instruction word (confirmed
by the adjacent W and I extractions via inst_get_field(inst,16,16) and
inst_get_field(inst,17,19)). Replace the open-coded logic with the standard
sign_extend32(inst & 0xfff, 11), which correctly performs two's-complement
sign extension from 12 bits to 32 bits.

Fixes: 831317b605e7 ("KVM: PPC: Implement Paired Single emulation")
Cc: stable@vger.kernel.org
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_paired_singles.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_paired_singles.c b/arch/powerpc/kvm/book3s_paired_singles.c
index bc39c76c9d9f..532f96293de0 100644
--- a/arch/powerpc/kvm/book3s_paired_singles.c
+++ b/arch/powerpc/kvm/book3s_paired_singles.c
@@ -479,12 +479,7 @@ static bool kvmppc_inst_is_paired_single(struct kvm_vcpu *vcpu, u32 inst)
 
 static int get_d_signext(u32 inst)
 {
-	int d = inst & 0x8ff;
-
-	if (d & 0x800)
-		return -(d & 0x7ff);
-
-	return (d & 0x7ff);
+	return sign_extend32(inst & 0xfff, 11);
 }
 
 static int kvmppc_ps_three_in(struct kvm_vcpu *vcpu, bool rc,
-- 
2.54.0 (Apple Git-157)

Re: [RESEND PATCH 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[Madhavan Srinivasan]

On Mon, 28 Sep 2026 17:58:34 +0530, Amit Machhiwal wrote:
> This series addresses a few bug fixes across KVM PPC Book3S HV
> locking/synchronization and paired-single instruction decoding:
> 
> Patches 1 & 2 fix synchronization and preemption issues introduced in
> commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for
> HPT hcalls and page faults"):
> 
> [...]

Applied to powerpc/next.

[1/3] KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls
      https://git.kernel.org/powerpc/c/61a39797bb136243c48b46e2586d8a9e2cea6a89
[2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users
      https://git.kernel.org/powerpc/c/abbbe5ff83e7c9ed3a4d61b73011f1df7bf87907
[3/3] KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single D-form
      https://git.kernel.org/powerpc/c/31895b0ff96778b524c74b43a108090581271603

cheers