altp2m: Drop p2m_altp2m_check() stubs on non-x86, move prototype, and guard uses

Remove the no-op stubs from the ARM, PPC, and RISC‑V p2m headers and stop
providing a stub in arch/x86/include/asm/p2m.h.

Declare p2m_altp2m_check() in xen/include/xen/p2m-common.h and gate all
call sites with CONFIG_ALTP2M:
 - wrap the fast_single_step block in hvm/monitor.c with #ifdef CONFIG_ALTP2M
   (IS_ENABLED(CONFIG_ALTP2M) is not used here, because in the subsequent commit
   hvm_vcpu::fast_single_step will be guarded by CONFIG_ALTP2M)
 - make the vm_event.c path conditional via IS_ENABLED(CONFIG_ALTP2M)

No functional change intended: on builds without ALTP2M the calls are
compiled out; on builds with ALTP2M behavior is unchanged.

Signed-off-by: Petr Beneš <w1benny@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com> # arm
Acked-by: Tamas K Lengyel <tamas@tklengyel.com> # vm_event/monitor

xen/arch/arm/include/asm/p2m.h

@@ -180,12 +180,6 @@ static inline bool arch_acquire_resource_check(struct domain *d)
     return true;
 }
 
-static inline
-void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
-{
-    /* Not supported on ARM. */
-}
-
 /*
  * Helper to restrict "p2m_ipa_bits" according the external entity
  * (e.g. IOMMU) requirements.

xen/arch/ppc/include/asm/p2m.h

@@ -88,9 +88,4 @@ static inline bool arch_acquire_resource_check(struct domain *d)
     return false;
 }
 
-static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
-{
-    /* Not supported on PPC. */
-}
-
 #endif /* __ASM_PPC_P2M_H__ */

xen/arch/riscv/include/asm/p2m.h

@@ -88,11 +88,6 @@ static inline bool arch_acquire_resource_check(struct domain *d)
     return false;
 }
 
-static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
-{
-    /* Not supported on RISCV. */
-}
-
 #endif /* ASM__RISCV__P2M_H */
 
 /*

xen/arch/x86/hvm/monitor.c

@@ -178,6 +178,7 @@ int hvm_monitor_debug(unsigned long rip, enum hvm_monitor_debug_type type,
         break;
 
     case HVM_MONITOR_SINGLESTEP_BREAKPOINT:
+#ifdef CONFIG_ALTP2M
         if ( curr->arch.hvm.fast_single_step.enabled )
         {
             p2m_altp2m_check(curr, curr->arch.hvm.fast_single_step.p2midx);
@@ -186,6 +187,7 @@ int hvm_monitor_debug(unsigned long rip, enum hvm_monitor_debug_type type,
             curr->arch.hvm.fast_single_step.p2midx = 0;
             return 0;
         }
+#endif
         if ( !ad->monitor.singlestep_enabled )
             return 0;
         req.reason = VM_EVENT_REASON_SINGLESTEP;

xen/arch/x86/include/asm/p2m.h

@@ -962,17 +962,11 @@ int p2m_altp2m_propagate_change(struct domain *d, gfn_t gfn,
 /* Set a specific p2m view visibility */
 int p2m_set_altp2m_view_visibility(struct domain *d, unsigned int altp2m_idx,
                                    uint8_t visible);
+
 #else /* !CONFIG_HVM */
 struct p2m_domain *p2m_get_altp2m(struct vcpu *v);
 #endif /* CONFIG_HVM */
 
-#ifdef CONFIG_ALTP2M
-/* Check to see if vcpu should be switched to a different p2m. */
-void p2m_altp2m_check(struct vcpu *v, uint16_t idx);
-#else
-static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx) {}
-#endif
-
 /* p2m access to IOMMU flags */
 static inline unsigned int p2m_access_to_iommu_flags(p2m_access_t p2ma)
 {

xen/common/vm_event.c

@@ -431,7 +431,8 @@ static int vm_event_resume(struct domain *d, struct vm_event_domain *ved)
             vm_event_toggle_singlestep(d, v, &rsp);
 
             /* Check for altp2m switch */
-            if ( rsp.flags & VM_EVENT_FLAG_ALTERNATE_P2M )
+            if ( IS_ENABLED(CONFIG_ALTP2M) &&
+                 rsp.flags & VM_EVENT_FLAG_ALTERNATE_P2M )
                 p2m_altp2m_check(v, rsp.altp2m_idx);
 
             if ( rsp.flags & VM_EVENT_FLAG_SET_REGISTERS )

xen/include/xen/p2m-common.h

@@ -24,6 +24,9 @@ int unmap_mmio_regions(struct domain *d,
                        unsigned long nr,
                        mfn_t mfn);
 
+/* Check to see if vcpu should be switched to a different p2m. */
+void p2m_altp2m_check(struct vcpu *v, uint16_t idx);
+
 /*
  * Populate-on-Demand
  */