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

["Michael S. Tsirkin" <mst@redhat.com>]

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 <peterx@redhat.com>

Paolo, if you can ack this one, I can merge the rest.

> ---
>  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