linux-kernel - Re: [PATCH v1] x86/hyperv: Reserve 3 interrupt vectors used exclusively by mshv

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Message-ID: <e6d26515-4ee7-4b1c-ad45-ca378330eed3@zytor.com>
Date: Tue, 10 Feb 2026 11:41:19 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Wei Liu <wei.liu@...nel.org>, Mukesh Rathor <mrathor@...ux.microsoft.com>
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        kys@...rosoft.com, haiyangz@...rosoft.com, decui@...rosoft.com,
        longli@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org
Subject: Re: [PATCH v1] x86/hyperv: Reserve 3 interrupt vectors used
 exclusively by mshv

On 2026-01-14 23:25, Wei Liu wrote:
> 
> I briefly looked up FRED and checked the code. I understand that once it
> is enabled, Linux kernel doesn't setup the IDT anymore (code in
> arch/x86/kernel/traps.c).
> 
> My question is, do we need to do anything when FRED is enabled?
> 

Assuming you don't need them handled in any specific way, then you don't. INT
instructions are treated completely separately from hardware interrupts in
FRED, and so they cannot be confused.

By default they will emulate a #GP(0) just as if an INT instruction had been
executed in user space with the DPL of the corresponding interrupt gate < 3;
this is currently the case for interrupt vectors other than 3, 4, and 0x80
(which are handled in fred_intx).

Any INT instruction in kernel space will end up in fred_bad_type().

	-hpa