RISC-V: KVM: Disable vstimecmp before exiting to user-space

If VS-timer expires when no VCPU running on a host CPU then WFI
executed by such host CPU will be effective NOP resulting in no
power savings. This is as-per RISC-V Privileged specificaiton
which says: "WFI is also required to resume execution for locally
enabled interrupts pending at any privilege level, regardless of
the global interrupt enable at each privilege level."

To address the above issue, vstimecmp CSR must be set to -1UL over
here when VCPU is scheduled-out or exits to user space.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: 8f5cb44b ("RISC-V: KVM: Support sstc extension")
Fixes: cea8896b ("RISC-V: KVM: Fix kvm_riscv_vcpu_timer_pending() for Sstc")
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2112578


Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://lore.kernel.org/r/20250707035345.17494-2-apatel@ventanamicro.com


Signed-off-by: Anup Patel <anup@brainfault.org>