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Re: [PATCH v3 2/2] ppc/spapr: Initialize max_cpus limit to SPAPR_IRQ_NR_


From: Cédric Le Goater
Subject: Re: [PATCH v3 2/2] ppc/spapr: Initialize max_cpus limit to SPAPR_IRQ_NR_IPIS.
Date: Thu, 23 Nov 2023 09:51:20 +0100
User-agent: Mozilla Thunderbird

On 11/23/23 06:57, Harsh Prateek Bora wrote:
Initialize the machine specific max_cpus limit as per the maximum range
of CPU IPIs available. Keeping between 4096 to 8192 will throw IRQ not
free error due to XIVE/XICS 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>


Reviewed-by: Cédric Le Goater <clg@kaod.org>

Thanks,

C.



---
  hw/ppc/spapr.c | 9 +++------
  1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index df09aa9d6a..222d926f46 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 the range of CPU IPIs available.
       */
-    mc->max_cpus = INT32_MAX;
+    mc->max_cpus = SPAPR_IRQ_NR_IPIS;
mc->no_parallel = 1;
      mc->default_boot_order = "";




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