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

[Grygorii Strashko <grygorii_strashko@epam.com>]

On 29.10.25 20:26, Teddy Astie wrote:
> 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@vates.tech>
> ---
> 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(-)
> 
> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
> index ad1553c5e9..01b41d93c0 100644
> --- a/docs/man/xl.cfg.5.pod.in
> +++ b/docs/man/xl.cfg.5.pod.in
> @@ -3198,6 +3198,13 @@ option.
>   
>   If using this option is necessary to fix an issue, please report a bug.
>   
> +=item B<force_x2apic=BOOLEAN>
> +
> +Force the LAPIC in x2APIC mode and prevent the guest from disabling
> +it or switching to xAPIC mode.
> +
> +This option is disabled by default.
> +
>   =back
>   
>   =head1 SEE ALSO

[...]

>   
> diff --git a/xen/arch/x86/include/asm/domain.h b/xen/arch/x86/include/asm/domain.h
> index 5df8c78253..771992d156 100644
> --- a/xen/arch/x86/include/asm/domain.h
> +++ b/xen/arch/x86/include/asm/domain.h
> @@ -509,6 +509,8 @@ struct arch_domain
>   #define has_pirq(d)        (!!((d)->arch.emulation_flags & X86_EMU_USE_PIRQ))
>   #define has_vpci(d)        (!!((d)->arch.emulation_flags & X86_EMU_VPCI))
>   
> +#define has_force_x2apic(d) ((d)->arch.hvm.force_x2apic)

Would it be possible for you to consider having Kconfig option to make
such configuration global, static?
   

-- 
Best regards,
-grygorii