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

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

On 11/22/23 14:06, Cédric Le Goater wrote:
> On 11/22/23 08:48, Harsh Prateek Bora wrote:
>> 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.
> 
> The IRQ number space is defined in include/hw/ppc/spapr_irq.h. XICS and
> XIVE have the same IRQ number space, it is not a XIVE limitation. It
> is how we organized interrupt numbers in the pseries-3.1 machine.
> 
> SPAPR_XIRQ_BASE defines an offset, at which the device IRQ numbers
> start, and below that offset, the range of IRQ numbers is reserved
> for IPIs. An assumption is made on the fact the both ranges, IPIs and
> devices, are contiguous and there is a little shortcut being done with
> the SPAPR_XIRQ_BASE define.
> 
> hw/ppc/spapr_irq.c:        qdev_prop_set_uint32(dev, "nr-irqs", 
> smc->nr_xirqs + SPAPR_XIRQ_BASE);
> hw/ppc/spapr_irq.c:                                      smc->nr_xirqs + 
> SPAPR_XIRQ_BASE);
> 
> This should use a SPAPR_NR_IPIS define (like we have a SPAPR_NR_XIRQS
> define) instead, which could be used to define mc->max_cpus like we
> define smc->nr_xirqs.
> 

Thanks Cedric for your review comments.
I have posted a v2 incorporating your suggestion.

regards,
Harsh
> Thanks,
> 
> C.
> 
> 
>> 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
>>
>