[PATCH] ppc/spapr: Initialize max_cpus limit to an allowed usable limit.

[Harsh Prateek Bora <harshpb@linux.ibm.com>]

Initialize the machine specific max_cpus limit to a usable limit 4096.
Keeping between 4096 to 8192 will throw IRQ not free error due to XIVE
limitation and keeping beyond 8192 will hit assert in tcg_region_init
or spapr_xive_claim_irq.

Logs:

Without patch fix:

[root@host build]# qemu-system-ppc64 -accel tcg -smp 10,maxcpus=4097
qemu-system-ppc64: IRQ 4096 is not free
[root@host build]#

On LPAR:
[root@host build]# qemu-system-ppc64 -accel tcg -smp 10,maxcpus=8193
**
ERROR:../tcg/region.c:774:tcg_region_init: assertion failed:
(region_size >= 2 * page_size)
Bail out! ERROR:../tcg/region.c:774:tcg_region_init: assertion failed:
(region_size >= 2 * page_size)
Aborted (core dumped)
[root@host build]#

On x86:
[root@host build]# qemu-system-ppc64 -accel tcg -smp 10,maxcpus=8193
qemu-system-ppc64: ../hw/intc/spapr_xive.c:596: spapr_xive_claim_irq:
Assertion `lisn < xive->nr_irqs' failed.
Aborted (core dumped)
[root@host build]#

With patch fix:
[root@host build]# qemu-system-ppc64 -accel tcg -smp 10,maxcpus=4097
qemu-system-ppc64: Invalid SMP CPUs 4097. The max CPUs supported by
machine 'pseries-8.2' is 4096
[root@host build]#

Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
---
 hw/ppc/spapr.c         | 9 +++------
 include/hw/ppc/spapr.h | 1 +
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index df09aa9d6a..1995949ea5 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -4647,13 +4647,10 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
     mc->block_default_type = IF_SCSI;
 
     /*
-     * Setting max_cpus to INT32_MAX. Both KVM and TCG max_cpus values
-     * should be limited by the host capability instead of hardcoded.
-     * max_cpus for KVM guests will be checked in kvm_init(), and TCG
-     * guests are welcome to have as many CPUs as the host are capable
-     * of emulate.
+     * While KVM determines max cpus in kvm_init() using kvm_max_vcpus(),
+     * In TCG the limit is restricted by max-irqs setup by XIVE which is 4096.
      */
-    mc->max_cpus = INT32_MAX;
+    mc->max_cpus = SPAPR_MAX_CPUS;
 
     mc->no_parallel = 1;
     mc->default_boot_order = "";
diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
index e91791a1a9..210849a494 100644
--- a/include/hw/ppc/spapr.h
+++ b/include/hw/ppc/spapr.h
@@ -23,6 +23,7 @@ typedef struct SpaprPendingHpt SpaprPendingHpt;
 
 typedef struct Vof Vof;
 
+#define SPAPR_MAX_CPUS          4096
 #define HPTE64_V_HPTE_DIRTY     0x0000000000000040ULL
 #define SPAPR_ENTRY_POINT       0x100
 
-- 
2.39.3