hw/riscv: virt: Increase maximum number of allowed CPUs

To facilitate software development of RISC-V systems with large number
of HARTs, we increase the maximum number of allowed CPUs to 512 (2^9).

We also add a detailed source level comments about limit defines which
impact the physical address space utilization.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-id: 20220204174700.534953-24-anup@brainfault.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

hw/riscv/virt.c

@@ -45,6 +45,16 @@
 #include "hw/pci-host/gpex.h"
 #include "hw/display/ramfb.h"
 
+/*
+ * The virt machine physical address space used by some of the devices
+ * namely ACLINT, PLIC, APLIC, and IMSIC depend on number of Sockets,
+ * number of CPUs, and number of IMSIC guest files.
+ *
+ * Various limits defined by VIRT_SOCKETS_MAX_BITS, VIRT_CPUS_MAX_BITS,
+ * and VIRT_IRQCHIP_MAX_GUESTS_BITS are tuned for maximum utilization
+ * of virt machine physical address space.
+ */
+
 #define VIRT_IMSIC_GROUP_MAX_SIZE      (1U << IMSIC_MMIO_GROUP_MIN_SHIFT)
 #if VIRT_IMSIC_GROUP_MAX_SIZE < \
     IMSIC_GROUP_SIZE(VIRT_CPUS_MAX_BITS, VIRT_IRQCHIP_MAX_GUESTS_BITS)

include/hw/riscv/virt.h

@@ -24,7 +24,7 @@
 #include "hw/block/flash.h"
 #include "qom/object.h"
 
-#define VIRT_CPUS_MAX_BITS             3
+#define VIRT_CPUS_MAX_BITS             9
 #define VIRT_CPUS_MAX                  (1 << VIRT_CPUS_MAX_BITS)
 #define VIRT_SOCKETS_MAX_BITS          2
 #define VIRT_SOCKETS_MAX               (1 << VIRT_SOCKETS_MAX_BITS)