Re: [PATCH v2] x86/hyperv: Reserve 3 interrupt vectors used exclusively by mshv

[Wei Liu <wei.liu@kernel.org>]

On Fri, Feb 20, 2026 at 05:14:26PM +0000, Michael Kelley wrote:
> From: Mukesh R <mrathor@linux.microsoft.com> Sent: Tuesday, February 17, 2026 3:12 PM
> > 
> > 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 for debug purposes.
> > 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>
> > ---
> > 
> > v1: Add ifndef CONFIG_X86_FRED (thanks hpa)
> > v2: replace ifndef with cpu_feature_enabled() (thanks hpa and tglx)
> > 
> >  arch/x86/kernel/cpu/mshyperv.c | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index 579fb2c64cfd..88ca127dc6d4 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -478,6 +478,28 @@ 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 either crash or hang or attempt to 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 (cpu_feature_enabled(X86_FEATURE_FRED))
> > +		return;
> > +
> > +	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();
> > +
> > +	pr_info("Hyper-V:reserve vectors: %d %d %d\n", HYPERV_DBG_ASSERT_VECTOR,
> > +		HYPERV_DBG_SERVICE_VECTOR, HYPERV_DBG_FASTFAIL_VECTOR);
> 
> I'm a little late to the party here, but I've always seen Intel interrupt vectors
> displayed as 2-digit hex numbers. This info message is displaying decimal,
> which is atypical and will probably be confusing.

Noted. The pull request to Linus has been sent. We will change the
format in a follow up patch.

Wei