Re: [PATCH v2 03/24] hw/xen: select kernel mode for per-vCPU event channel upcall vector

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 19/10/2023 16:39, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> A guest which has configured the per-vCPU upcall vector may set the
> HVM_PARAM_CALLBACK_IRQ param to fairly much anything other than zero.
>
> For example, Linux v6.0+ after commit b1c3497e604 ("x86/xen: Add support
> for HVMOP_set_evtchn_upcall_vector") will just do this after setting the
> vector:
>
>         /* Trick toolstack to think we are enlightened. */
>         if (!cpu)
>                 rc = xen_set_callback_via(1);
>
> That's explicitly setting the delivery to GSI#1, but it's supposed to be
> overridden by the per-vCPU vector setting. This mostly works in Qemu
> *except* for the logic to enable the in-kernel handling of event channels,
> which falsely determines that the kernel cannot accelerate GSI delivery
> in this case.
>
> Add a kvm_xen_has_vcpu_callback_vector() to report whether vCPU#0 has
> the vector set, and use that in xen_evtchn_set_callback_param() to
> enable the kernel acceleration features even when the param *appears*
> to be set to target a GSI.
>
> Preserve the Xen behaviour that when HVM_PARAM_CALLBACK_IRQ is set to
> *zero* the event channel delivery is disabled completely. (Which is
> what that bizarre guest behaviour is working round in the first place.)
>
> Fixes: 91cce756179 ("hw/xen: Add xen_evtchn device for event channel emulation")
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   hw/i386/kvm/xen_evtchn.c  | 6 ++++++
>   include/sysemu/kvm_xen.h  | 1 +
>   target/i386/kvm/xen-emu.c | 7 +++++++
>   3 files changed, 14 insertions(+)
Reviewed-by: Paul Durrant <paul@xxxxxxx>