Re: [SeaBIOS PATCH] xen: require Xen info structure at 0x1000 to detect Xen

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 20/01/2023 11:33, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> When running under Xen, hvmloader places a table at 0x1000 with the e820
> information and BIOS tables. If this isn't present, SeaBIOS will
> currently panic.
>
> We now have support for running Xen guests natively in QEMU/KVM, which
> boots SeaBIOS directly instead of via hvmloader, and does not provide
> the same structure.
>
> As it happens, this doesn't matter on first boot. because although we
> set PlatformRunningOn to PF_QEMU|PF_XEN, reading it back again still
> gives zero. Presumably because in true Xen, this is all already RAM. But
> in QEMU with a faithfully-emulated PAM config in the host bridge, it's
> still in ROM mode at this point so we don't see what we've just written.
>
> On reboot, however, the region *is* set to RAM mode and we do see the
> updated value of PlatformRunningOn, do manage to remember that we've
> detected Xen in CPUID, and hit the panic.
>
> It's not trivial to detect QEMU vs. real Xen at the time xen_preinit()
> runs, because it's so early. We can't even make a XENVER_extraversion
> hypercall to look for hints, because we haven't set up the hypercall
> page (and don't have an allocator to give us a page in which to do so).
>
> So just make Xen detection contingent on the info structure being
> present. If it wasn't, we were going to panic anyway. That leaves us
> taking the standard QEMU init path for Xen guests in native QEMU,
> which is just fine.
>
> Untested on actual Xen but ObviouslyCorrect™.
Works for me...

Tested-by: Paul Durrant <paul@xxxxxxx>

> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>   src/fw/xen.c | 45 ++++++++++++++++++++++++++++++++-------------
>   1 file changed, 32 insertions(+), 13 deletions(-)
Also...

Reviewed-by: Paul Durrant <paul@xxxxxxx>