Re: [Qemu-devel] [PATCH 1/4] kvm: let split be optional for kvm_arch_irqchip_create

[Paolo Bonzini]

On 19/12/18 16:53, Michael S. Tsirkin wrote:
> On Wed, Dec 19, 2018 at 04:50:35PM +0800, Peter Xu wrote:
>> This patch allows the kvm_arch_irqchip_create() to return 0 if the
>> split irqchip is specified but not forced by the user.  Also, modify
>> kvm_irqchip_create() similiarly.
>>
>> This patch should have no functional change for existing code since
>> currently if split is specified it must be forced by the user so we'll
>> always have machine_kernel_irqchip_required() returns true. However it
>> could potentially be used in follow up patches when we want to turn
>> split kernel irqchip as default for QEMU 4.0 which could trigger the
>> case that kernel_irqchip_required=N while kernel_irqchip_split=Y. When
>> with that, we'll first try with split irqchip, and falls back to
>> normal kernel irqchip when split capability is not provided by the
>> kernel.
>>
>> This brings us benefit that we can even run a default QEMU 4.0 on old
>> kernels that does not support split irqchip (<4.4) but at the same
>> time enable split irqchip for new kernels (>=4.4) as default.
>>
>> Signed-off-by: Peter Xu <address@hidden>
> 
> Paolo, if you can ack this one, I can merge the rest.

If I understand the code well, there is no change needed in the rest of
the code; having the semantics I asked for simply requires dropping this
patch.

However, the commit messages need some adjustment.

Paolo

> 
>> ---
>>  accel/kvm/kvm-all.c | 3 ++-
>>  target/i386/kvm.c   | 6 +++---
>>  2 files changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>> index 4880a05399..b008364041 100644
>> --- a/accel/kvm/kvm-all.c
>> +++ b/accel/kvm/kvm-all.c
>> @@ -1468,7 +1468,8 @@ static void kvm_irqchip_create(MachineState *machine, 
>> KVMState *s)
>>       * in-kernel irqchip for us */
>>      ret = kvm_arch_irqchip_create(machine, s);
>>      if (ret == 0) {
>> -        if (machine_kernel_irqchip_split(machine)) {
>> +        if (machine_kernel_irqchip_required(machine) &&
>> +            machine_kernel_irqchip_split(machine)) {
>>              perror("Split IRQ chip mode not supported.");
>>              exit(1);
>>          } else {
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index 739cf8c8ea..8f919f8f9f 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -3685,9 +3685,9 @@ int kvm_arch_irqchip_create(MachineState *ms, KVMState 
>> *s)
>>      if (machine_kernel_irqchip_split(ms)) {
>>          ret = kvm_vm_enable_cap(s, KVM_CAP_SPLIT_IRQCHIP, 0, 24);
>>          if (ret) {
>> -            error_report("Could not enable split irqchip mode: %s",
>> -                         strerror(-ret));
>> -            exit(1);
>> +            assert(ret < 0);
>> +            /* If split not required, return 0 instead to retry */
>> +            return machine_kernel_irqchip_required(ms) ? ret : 0;
>>          } else {
>>              DPRINTF("Enabled KVM_CAP_SPLIT_IRQCHIP\n");
>>              kvm_split_irqchip = true;
>> -- 
>> 2.17.1