RISC-V: Allow interrupt controllers to claim interrupts

We can't allow the supervisor to control SEIP as this would allow the
supervisor to clear a pending external interrupt which will result in
lost a interrupt in the case a PLIC is attached. The SEIP bit must be
hardware controlled when a PLIC is attached.

This logic was previously hard-coded so SEIP was always masked even
if no PLIC was attached. This patch adds riscv_cpu_claim_interrupts
so that the PLIC can register control of SEIP. In the case of models
without a PLIC (spike), the SEIP bit remains software controlled.

This interface allows for hardware control of supervisor timer and
software interrupts by other interrupt controller models.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

hw/riscv/sifive_plic.c

@@ -23,6 +23,7 @@
 #include "qemu/error-report.h"
 #include "hw/sysbus.h"
 #include "target/riscv/cpu.h"
+#include "sysemu/sysemu.h"
 #include "hw/riscv/sifive_plic.h"
 
 #define RISCV_DEBUG_PLIC 0
@@ -431,6 +432,7 @@ static void sifive_plic_irq_request(void *opaque, int irq, int level)
 static void sifive_plic_realize(DeviceState *dev, Error **errp)
 {
     SiFivePLICState *plic = SIFIVE_PLIC(dev);
+    int i;
 
     memory_region_init_io(&plic->mmio, OBJECT(dev), &sifive_plic_ops, plic,
                           TYPE_SIFIVE_PLIC, plic->aperture_size);
@@ -443,6 +445,19 @@ static void sifive_plic_realize(DeviceState *dev, Error **errp)
     plic->enable = g_new0(uint32_t, plic->bitfield_words * plic->num_addrs);
     sysbus_init_mmio(SYS_BUS_DEVICE(dev), &plic->mmio);
     qdev_init_gpio_in(dev, sifive_plic_irq_request, plic->num_sources);
+
+    /* We can't allow the supervisor to control SEIP as this would allow the
+     * supervisor to clear a pending external interrupt which will result in
+     * lost a interrupt in the case a PLIC is attached. The SEIP bit must be
+     * hardware controlled when a PLIC is attached.
+     */
+    for (i = 0; i < smp_cpus; i++) {
+        RISCVCPU *cpu = RISCV_CPU(qemu_get_cpu(i));
+        if (riscv_cpu_claim_interrupts(cpu, MIP_SEIP) < 0) {
+            error_report("SEIP already claimed");
+            exit(1);
+        }
+    }
 }
 
 static void sifive_plic_class_init(ObjectClass *klass, void *data)

target/riscv/cpu.h

@@ -140,6 +140,7 @@ struct CPURISCVState {
      * mip is 32-bits to allow atomic_read on 32-bit hosts.
      */
     uint32_t mip;
+    uint32_t miclaim;
 
     target_ulong mie;
     target_ulong mideleg;
@@ -266,6 +267,7 @@ void riscv_cpu_list(FILE *f, fprintf_function cpu_fprintf);
 #define cpu_mmu_index riscv_cpu_mmu_index
 
 #ifndef CONFIG_USER_ONLY
+int riscv_cpu_claim_interrupts(RISCVCPU *cpu, uint32_t interrupts);
 uint32_t riscv_cpu_update_mip(RISCVCPU *cpu, uint32_t mask, uint32_t value);
 #define BOOL_TO_MASK(x) (-!!(x)) /* helper for riscv_cpu_update_mip value */
 #endif

target/riscv/cpu_helper.c

@@ -72,6 +72,17 @@ bool riscv_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
 
 #if !defined(CONFIG_USER_ONLY)
 
+int riscv_cpu_claim_interrupts(RISCVCPU *cpu, uint32_t interrupts)
+{
+    CPURISCVState *env = &cpu->env;
+    if (env->miclaim & interrupts) {
+        return -1;
+    } else {
+        env->miclaim |= interrupts;
+        return 0;
+    }
+}
+
 /* iothread_mutex must be held */
 uint32_t riscv_cpu_update_mip(RISCVCPU *cpu, uint32_t mask, uint32_t value)
 {

target/riscv/csr.c

@@ -550,16 +550,10 @@ static int rmw_mip(CPURISCVState *env, int csrno, target_ulong *ret_value,
                    target_ulong new_value, target_ulong write_mask)
 {
     RISCVCPU *cpu = riscv_env_get_cpu(env);
-    target_ulong mask = write_mask & delegable_ints;
+    /* Allow software control of delegable interrupts not claimed by hardware */
+    target_ulong mask = write_mask & delegable_ints & ~env->miclaim;
     uint32_t old_mip;
 
-    /* We can't allow the supervisor to control SEIP as this would allow the
-     * supervisor to clear a pending external interrupt which will result in
-     * lost a interrupt in the case a PLIC is attached. The SEIP bit must be
-     * hardware controlled when a PLIC is attached. This should be an option
-     * for CPUs with software-delegated Supervisor External Interrupts. */
-    mask &= ~MIP_SEIP;
-
     if (mask) {
         qemu_mutex_lock_iothread();
         old_mip = riscv_cpu_update_mip(cpu, mask, (new_value & mask));