linux-kernel - Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in _

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Message-ID: <CABgObfZQ2n6PB0i4Uc6k4Rm9bVESt0aafOcdLzW4hwX3sN-ExA@mail.gmail.com>
Date: Thu, 1 May 2025 20:33:57 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com, 
	wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, seanjc@...gle.com, 
	ardb@...nel.org, kees@...nel.org, Arnd Bergmann <arnd@...db.de>, 
	gregkh@...uxfoundation.org, jpoimboe@...nel.org, linux-hyperv@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, linux-efi@...r.kernel.org, 
	samitolvanen@...gle.com, ojeda@...nel.org
Subject: Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in
 __nocfi functions

On Wed, Apr 30, 2025 at 1:26 PM Peter Zijlstra <peterz@...radead.org> wrote:
> Notably the KVM fastop emulation stuff -- which I've completely rewritten for
> this version -- the generated code doesn't look horrific, but is slightly more
> verbose. I'm running on the assumption that instruction emulation is not super
> performance critical these days of zero VM-exit VMs etc.

It's definitely going to be slower, but I guess it's okay these days.
It's really only somewhat hot with really old processors
(pre-Westmere) and only when running big real mode code.

Paolo

> KVM has another; the VMX interrupt injection stuff calls the IDT handler
> directly.  Is there an alternative? Can we keep a table of Linux functions
> slighly higher up the call stack (asm_\cfunc ?) and add CFI to those?
>
> HyperV hypercall page stuff, which I've previously suggested use direct calls,
> and which I've now converted (after getting properly annoyed with that code).
>
> Also available at:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/core
>
> Changes since v1:
>
>  - complete rewrite of the fastop stuff
>  - HyperV tweaks (Michael)
>  - objtool changes (Josh)
>
>
> [1] https://lkml.kernel.org/r/20250410154556.GB9003@noisy.programming.kicks-ass.net
> [2] https://lkml.kernel.org/r/20250410194334.GA3248459@google.com
>