riscv: kvm: Fix vstimecmp update hazard on RV32

On RV32, updating the 64-bit stimecmp (or vstimecmp) CSR requires two
separate 32-bit writes. A race condition exists if the timer triggers
during these two writes.

The RISC-V Privileged Specification (e.g., Section 3.2.1 for mtimecmp)
recommends a specific 3-step sequence to avoid spurious interrupts
when updating 64-bit comparison registers on 32-bit systems:

1. Set the low-order bits (stimecmp) to all ones (ULONG_MAX).
2. Set the high-order bits (stimecmph) to the desired value.
3. Set the low-order bits (stimecmp) to the desired value.

Current implementation writes the LSB first without ensuring a future
value, which may lead to a transient state where the 64-bit comparison
is incorrectly evaluated as "expired" by the hardware. This results in
spurious timer interrupts.

This patch adopts the spec-recommended 3-step sequence to ensure the
intermediate 64-bit state is never smaller than the current time.

Fixes: 8f5cb44 ("RISC-V: KVM: Support sstc extension")
Signed-off-by: Naohiko Shimizu <naohiko.shimizu@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://patch.msgid.link/20260104135938.524-3-naohiko.shimizu@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: pshimizu

arch/riscv/kvm/vcpu_timer.c

@@ -72,8 +72,9 @@ static int kvm_riscv_vcpu_timer_cancel(struct kvm_vcpu_timer *t)
 static int kvm_riscv_vcpu_update_vstimecmp(struct kvm_vcpu *vcpu, u64 ncycles)
 {
 #if defined(CONFIG_32BIT)
-	ncsr_write(CSR_VSTIMECMP, ncycles & 0xFFFFFFFF);
+	ncsr_write(CSR_VSTIMECMP,  ULONG_MAX);
 	ncsr_write(CSR_VSTIMECMPH, ncycles >> 32);
+	ncsr_write(CSR_VSTIMECMP, (u32)ncycles);
 #else
 	ncsr_write(CSR_VSTIMECMP, ncycles);
 #endif
@@ -307,8 +308,9 @@ void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu)
 		return;
 
 #if defined(CONFIG_32BIT)
-	ncsr_write(CSR_VSTIMECMP, (u32)t->next_cycles);
+	ncsr_write(CSR_VSTIMECMP, ULONG_MAX);
 	ncsr_write(CSR_VSTIMECMPH, (u32)(t->next_cycles >> 32));
+	ncsr_write(CSR_VSTIMECMP, (u32)(t->next_cycles));
 #else
 	ncsr_write(CSR_VSTIMECMP, t->next_cycles);
 #endif