RE: [RFC][PATCH v0] x86/hyperv: Reserve 3 interrupt vectors used exclusively by mshv

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michael Kelley <mhklinux@outlook.com> writes:

> From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Friday, January 2, 2026 7:55 AM
>> 
>> Mukesh Rathor <mrathor@linux.microsoft.com> writes:
>> 
>> > MSVC compiler used to compile the Microsoft Hyper-V hypervisor currently,
>> > has an assert intrinsic that uses interrupt vector 0x29 to create an
>> > exception. This will cause hypervisor to then crash and collect core. As
>> > such, if this interrupt number is assigned to a device by linux and the
>> > device generates it, hypervisor will crash. There are two other such
>> > vectors hard coded in the hypervisor, 0x2C and 0x2D.
>> >
>> > Fortunately, the three vectors are part of the kernel driver space, and
>> > that makes it feasible to reserve them early so they are not assigned
>> > later.
>> >
>> > Signed-off-by: Mukesh Rathor <mrathor@linux.microsoft.com>
>> > ---
>> >  arch/x86/kernel/cpu/mshyperv.c | 22 ++++++++++++++++++++++
>> >  1 file changed, 22 insertions(+)
>> >
>> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
>> > index 579fb2c64cfd..19d41f7434df 100644
>> > --- a/arch/x86/kernel/cpu/mshyperv.c
>> > +++ b/arch/x86/kernel/cpu/mshyperv.c
>> > @@ -478,6 +478,25 @@ int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
>> >  }
>> >  EXPORT_SYMBOL_GPL(hv_get_hypervisor_version);
>> >
>> > +/*
>> > + * Reserve vectors hard coded in the hypervisor. If used outside, the hypervisor
>> > + * will crash or hang or break into debugger.
>> > + */
>> > +static void hv_reserve_irq_vectors(void)
>> > +{
>> > +	#define HYPERV_DBG_FASTFAIL_VECTOR	0x29
>> > +	#define HYPERV_DBG_ASSERT_VECTOR	0x2C
>> > +	#define HYPERV_DBG_SERVICE_VECTOR	0x2D
>> > +
>> > +	if (test_and_set_bit(HYPERV_DBG_ASSERT_VECTOR, system_vectors) ||
>> > +	    test_and_set_bit(HYPERV_DBG_SERVICE_VECTOR, system_vectors) ||
>> > +	    test_and_set_bit(HYPERV_DBG_FASTFAIL_VECTOR, system_vectors))
>> > +		BUG();
>> 
>> Would it be less hackish to use sysvec_install() with a dummy handler
>> for all three vectors instead?
>
> It would be, but unfortunately, it doesn't work. sysvec_install() requires
> that the vector be >= FIRST_SYSTEM_VECTOR, and these vectors are not.
>

True; then maybe introduce a new API like sysvec_reserve() without the
limitation? What I'm personally afraid of is that looking at
sysvec_install() it already has an additional fred_install_sysvec()
which operates over its own sysvec_table and only does
idt_install_sysvec() when !cpu_feature_enabled(X86_FEATURE_FRED) -- and
this patch just plays with system_vectors directly. Maybe this is even
correct for now but I believe can be fragile in the future.

Ultimately, I think it's up to x86 maintainers to say whether they think
that playing with system_vectors outside of the core is OK and expected
or if a new, explicit API is preferable.

-- 
Vitaly