Re: [PATCH v2 0/6] x86/PVH: Dom0 building adjustments

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.09.2021 10:30, Jan Beulich wrote:
> The code building PVH Dom0 made use of sequences of P2M changes
> which are disallowed as of XSA-378. First of all population of the
> first Mb of memory needs to be redone. Then, largely as a
> workaround, checking introduced by XSA-378 needs to be slightly
> relaxed.
> 
> Note that with these adjustments I get Dom0 to start booting on my
> development system, but the Dom0 kernel then gets stuck. Since it
> was the first time for me to try PVH Dom0 in this context (see
> below for why I was hesitant), I cannot tell yet whether this is
> due further fallout from the XSA, or some further unrelated
> problem. Dom0's BSP makes it all the way through to entering the
> idle loop while all APs are still in VPF_down.

This last part of the mystery is now solved: By cloning from my PV
.config, I've inherited the X86_X2APIC=n that I have there. Yet
ACPI's MADT gets populated with only x2APIC entries when building
Dom0, which such a kernel would mostly ignore (except for logging).
IOW in a way this was indeed a missing select, except that what's
needed wouldn't quite work yet:

--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -83,6 +83,6 @@ config XEN_PVH
        bool "Xen PVH guest support"
        depends on XEN && XEN_PVHVM && ACPI
        select PVH
-       def_bool n
+       select X86_X2APIC if XEN_DOM0
        help
          Support for running as a Xen PVH guest.

This is because, as mentioned, XEN_DOM0 gets turned off when XEN_PV
is off. Whereas x2APIC isn't strictly needed for DomU afaics
(MADT gets populated with LAPIC entries only), so the "select" also
shouldn't be unconditional.

While likely no-one will really care, I'd like to note that this
effectively means a 32-bit Linux PVH Dom0 would be impossible, as
X86_X2APIC depends on X86_64. This may want reflecting in the
eventual adjustment to the XEN_DOM0 dependencies.

Btw, I've meanwhile also checked timers: They're active and get
updated while Dom0 is in this funny idle state.

Jan