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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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?

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