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

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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>




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