Re: [PATCH v2 2/2] ppc/spapr: Initialize max_cpus limit to SPAPR_NR_IPIS.

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

Hi Philippe,

On 11/22/23 16:46, Philippe Mathieu-Daudé wrote:
> Hi Harsh,
> 
> On 22/11/23 10:28, 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>
>> ---
>>   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..0de11a4458 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_NR_IPIS;
> 
> Is SPAPR_NR_IPIS also the upper limit for KVM?

In KVM mode, the limit is restricted to what is supported by KVM which 
is checked using kvm_ioctl via wrappers in kvm_init and appears to be 
evaluating to 2048. So, having a larger default works for both case.

regards,
Harsh

> 
>>       mc->no_parallel = 1;
>>       mc->default_boot_order = "";
>