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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/11/2025 10:35 am, Teddy Astie wrote:
> 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.

You don't need any new hypercalls.  Just set the state correctly in a
LAPIC record in libxg's vcpu_hvm().

~Andrew