Re: [PATCH v6 12/51] i386/xen: Implement SCHEDOP_poll and SCHEDOP_yield

[Paul Durrant]

On 10/01/2023 12:20, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> They both do the same thing and just call sched_yield. This is enough to
> stop the Linux guest panicking when running on a host kernel which doesn't
> intercept SCHEDOP_poll and lets it reach userspace.
>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

Reviewed-by: Paul Durrant <paul@xen.org>

... with some observations...

> ---
>   target/i386/kvm/xen-emu.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/target/i386/kvm/xen-emu.c b/target/i386/kvm/xen-emu.c
> index 5f2b55ef10..80005ea527 100644
> --- a/target/i386/kvm/xen-emu.c
> +++ b/target/i386/kvm/xen-emu.c
> @@ -227,6 +227,18 @@ static bool kvm_xen_hcall_sched_op(struct kvm_xen_exit 
> *exit, X86CPU *cpu,
>           err = schedop_shutdown(cs, arg);
>           break;
>   
> +    case SCHEDOP_poll:
> +        /*
> +         * Linux will panic if this doesn't work. Just yield; it's not
> +         * worth overthinking it because wWith event channel handling

Typo 'wWith'. Also possibly worth mentioning that the reason a yield is 
ok is because the semantics of the hypercall allow for spurious wake-up.

> +         * in KVM, the kernel will intercept this and it will never
> +         * reach QEMU anyway.
> +         */
> +    case SCHEDOP_yield:
> +        sched_yield();
> +        err = 0;
> +        break;
> +
>       default:
>           return false;
>       }