Re: [XTF PATCH v2 2/2] x86: Allow exiting QEMU in TCG/QEMU

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Oct 02, 2025 at 03:55:34PM +0200, Alejandro Vallejo wrote:
> If QEMU has a debug isa-debug-exit device, we can simply write to it
> to exit rather than spinning after a failed hypercall.
> 
> While at it, reorder an out-of-order include.
> 
> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
> ---
>  arch/x86/hvm/traps.c    | 16 +++++++++++++++-
>  arch/x86/pv/traps.c     |  5 +++++
>  common/lib.c            |  2 +-
>  common/report.c         |  8 +++++---
>  include/xtf/framework.h |  3 +++
>  5 files changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/hvm/traps.c b/arch/x86/hvm/traps.c
> index ad7b8cb..b8c4d0c 100644
> --- a/arch/x86/hvm/traps.c
> +++ b/arch/x86/hvm/traps.c
> @@ -1,5 +1,6 @@
> -#include <xtf/traps.h>
> +#include <xtf/hypercall.h>
>  #include <xtf/lib.h>
> +#include <xtf/traps.h>
>  
>  #include <arch/idt.h>
>  #include <arch/lib.h>
> @@ -139,6 +140,19 @@ void arch_init_traps(void)
>                 virt_to_gfn(__end_user_bss));
>  }
>  
> +void arch_shutdown(unsigned int reason)
> +{
> +    hypercall_shutdown(reason);

This relies on the hypercall page being poised with `ret`, which is
IMO fragile.  I would rather have it poisoned with `int3` and prevent
such stray accesses in the first place.

> +
> +    /*
> +     * Not running under Xen. Attempt exit via the QEMU ISA debug exit device on
> +     * its default port.
> +     *
> +     * QEMU's rc is (reason << 1) | 1, if "-device isa-debug-exit" is set.
> +     */
> +    outb(reason, 0x501);

That's kind of weird?  So even if we pass reason == 0, the exit code
from QEMU will be 1 (and error)?

Isn't there anyway to signal a clean shutdown, and hence QEMU exit
code being 0?

Thanks, Roger.