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

[Kevin O'Connor]

On Fri, Jan 20, 2023 at 11:33:19AM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> 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.

Thanks.  I committed this change.

-Kevin