RISC-V: KVM: Add perf sampling support for guests

KVM enables perf for guest via counter virtualization. However, the
sampling can not be supported as there is no mechanism to enabled
trap/emulate scountovf in ISA yet. Rely on the SBI PMU snapshot
to provide the counter overflow data via the shared memory.

In case of sampling event, the host first sets the guest's LCOFI
interrupt and injects to the guest via irq filtering mechanism defined
in AIA specification. Thus, ssaia must be enabled in the host in order
to use perf sampling in the guest. No other AIA dependency w.r.t kernel
is required.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-15-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/include/uapi/asm/kvm.h

@@ -167,6 +167,7 @@ enum KVM_RISCV_ISA_EXT_ID {
 	KVM_RISCV_ISA_EXT_ZFA,
 	KVM_RISCV_ISA_EXT_ZTSO,
 	KVM_RISCV_ISA_EXT_ZACAS,
+	KVM_RISCV_ISA_EXT_SSCOFPMF,
 	KVM_RISCV_ISA_EXT_MAX,
 };