[xen staging] xen/riscv: add vtimer context switch helpers

[patchbot@xxxxxxx]

commit 4313feae90068fb8fccafe401dd7976cbe117db8
Author:     Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
AuthorDate: Fri Mar 6 17:33:24 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 10 09:26:24 2026 +0100

    xen/riscv: add vtimer context switch helpers
    
    Introduce vtimer_ctxt_switch_from() and vtimer_ctxt_switch_to() to handle
    virtual timer state across vCPU context switches.
    
    At present, vtimer_ctxt_switch_from() is a no-op because the RISC-V SSTC
    extension, which provides a virtualization-aware timer, is not yet
    supported. Xen therefore relies the virtual (SBI-based) timer.
    
    The virtual timer uses Xen's internal timer infrastructure and must be
    associated with the pCPU on which the vCPU is currently running so that
    timer events can be delivered efficiently. As a result, 
vtimer_ctxt_switch_to()
    migrates the timer to the target pCPU when a vCPU is scheduled in.
    
    Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/riscv/include/asm/vtimer.h |  3 +++
 xen/arch/riscv/vtimer.c             | 15 +++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/xen/arch/riscv/include/asm/vtimer.h 
b/xen/arch/riscv/include/asm/vtimer.h
index 111863610a..b4d48d1a17 100644
--- a/xen/arch/riscv/include/asm/vtimer.h
+++ b/xen/arch/riscv/include/asm/vtimer.h
@@ -14,4 +14,7 @@ void vcpu_timer_destroy(struct vcpu *v);
 
 void vtimer_set_timer(struct vtimer *t, uint64_t ticks);
 
+void vtimer_ctxt_switch_from(struct vcpu *p);
+void vtimer_ctxt_switch_to(struct vcpu *n);
+
 #endif /* ASM__RISCV__VTIMER_H */
diff --git a/xen/arch/riscv/vtimer.c b/xen/arch/riscv/vtimer.c
index 32d142bcdf..afd8a53a73 100644
--- a/xen/arch/riscv/vtimer.c
+++ b/xen/arch/riscv/vtimer.c
@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <xen/bug.h>
 #include <xen/sched.h>
 #include <xen/timer.h>
 
@@ -69,3 +70,17 @@ void vtimer_set_timer(struct vtimer *t, const uint64_t ticks)
     migrate_timer(&t->timer, smp_processor_id());
     set_timer(&t->timer, expires);
 }
+
+void vtimer_ctxt_switch_from(struct vcpu *p)
+{
+    ASSERT(!is_idle_vcpu(p));
+
+    /* Nothing to do at the moment as SSTC isn't supported now. */
+}
+
+void vtimer_ctxt_switch_to(struct vcpu *n)
+{
+    ASSERT(!is_idle_vcpu(n));
+
+    migrate_timer(&n->arch.vtimer.timer, n->processor);
+}
--
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