Re: [RFC PATCH for-4.22] x86/hvm: Introduce force_x2apic flag

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

Le 29/10/2025 à 19:26, Teddy Astie a écrit :
> Introduce a new flag to force the x2APIC enabled and preventing a
> guest from switching back LAPIC to xAPIC mode.
>
> The semantics of this mode are based IA32_XAPIC_DISABLE_STATUS
> architectural MSR of Intel specification.
>
> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
> ---
> This feature can be useful for various reasons, starting with SEV as
> it is complicated (especially with SEV-ES) to handle MMIO, and legacy
> xAPIC is one thing that needs MMIO intercepts (and Linux uses it during
> boot unless x2APIC is initially enabled, even if it switches to
> x2apic afterward). It could also be interesting to reduce the attack
> surface of the hypervisor (by only exposing x2apic to the guest).
>
> As it can allow to have MMIO-less guest (using PVH), perhaps it can
> be enough for avoiding the problematic cases of virtualized INVLPGB
> (when we have it).
>
> In my testing, Linux, FreeBSD and PV-shim works fine with it; OVMF
> freezes for some reason, NetBSD doesn't support it (no x2apic support
> as Xen guest). HVM BIOS gets stuck at SeaBIOS as it expects booting
> with xAPIC.
>
> On Intel platforms, it would be better to expose the
> IA32_XAPIC_DISABLE_STATUS architectural MSR to advertise this to
> guest, but it's non-trivial as it needs to be properly exposed
> through IA32_ARCH_CAPABILITIES which is currently passed-through.
>
>   docs/man/xl.cfg.5.pod.in              |  7 +++++++
>   tools/libs/light/libxl_types.idl      |  1 +
>   tools/libs/light/libxl_x86.c          |  4 ++++
>   tools/xl/xl_parse.c                   |  1 +
>   xen/arch/x86/domain.c                 |  2 +-
>   xen/arch/x86/hvm/hvm.c                |  2 ++
>   xen/arch/x86/hvm/vlapic.c             | 23 ++++++++++++++++++++++-
>   xen/arch/x86/include/asm/domain.h     |  2 ++
>   xen/arch/x86/include/asm/hvm/domain.h |  3 +++
>   xen/include/public/arch-x86/xen.h     | 12 +++++++++++-
>   10 files changed, 54 insertions(+), 3 deletions(-)
>

I guess for now, it would be preferable overall to :
- just add a way to enable it by default, not lock in it in x2apic mode
- the ability to lock it down (i.e disable xAPIC at compile time) could
be introduced separately

I'm not completely decided on the naming of the option, maybe something
like :
x2apic_mode = <default> | <pre_enable> (or just enable ?)

`default` will keep the current behavior, or force x2apic if xAPIC is
disabled at compile time; `pre_enable` will enable it by default, but OS
may be able to go back to xAPIC mode if supported.

Teddy


--
Teddy Astie | Vates XCP-ng Developer

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