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Re: [PULL 0/1] xen queue 2020-08-03

From: Peter Maydell
Subject: Re: [PULL 0/1] xen queue 2020-08-03
Date: Mon, 3 Aug 2020 19:33:10 +0100

On Mon, 3 Aug 2020 at 17:53, Anthony PERARD <anthony.perard@citrix.com> wrote:
> ----------------------------------------------------------------
> xen patches
> bug fix
> ----------------------------------------------------------------
> Philippe Mathieu-Daudé (1):
>       accel/xen: Fix xen_enabled() behavior on target-agnostic objects

This fails to compile, all hosts :-(

  LINK    avr-softmmu/qemu-system-avr
../migration/savevm.o: In function `qemu_loadvm_section_start_full':
undefined reference to `xen_enabled'
collect2: error: ld returned 1 exit status
[ditto other softmmu link attempts]

Similar but different on OSX:

  LINK    aarch64-softmmu/qemu-system-aarch64
Undefined symbols for architecture x86_64:
  "_xen_enabled", referenced from:
      _msix_is_masked in msix.o
      _msix_write_config in msix.o
      _msix_handle_mask_update in msix.o
      _msix_init in msix.o
      _msix_notify in msix.o
      _msix_reset in msix.o
      _msix_set_vector_notifiers in msix.o
ld: symbol(s) not found for architecture x86_64

Fails in all the BSD VM builds as well, so I don't think this
is related to incremental vs from-clean builds.

I think the failure here is only on builds where
CONFIG_XEN is not set by configure (which for my merge
build setup appears to be all of them, I think).
In that case when we build a target-agnostic object
it gets an external reference to the xen_enabled()
function, but because Xen is disabled we don't compile
accel/xen/ and so there is no actual implementation of
the function in the link.

This doesn't happen for KVM because there the
CONFIG_KVM_IS_POSSIBLE version of kvm_enabled()
is a #define:
#define kvm_enabled()           (kvm_allowed)
and we provide both the "real" version of the
'bool kvm_allowed' in accel/kvm/kvm-all.c and also
the stub version in accel/stubs/kvm-stub.c which will
be used in configurations thta don't build accel/kvm.

I think then that the fix for this build failure is to
 (1) add a stub version of xen_allowed() to accel/stubs/xen-stub.c
     (which would always return false)

 (2) follow KVM more closely, and have xen_allowed() be
     a #define to (xen_allowed), with a (non-static)
     bool xen_allowed in accel/xen/xen-all.c and another
     one in accel/stubs/xen-stub.c

-- PMM

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