linux-kernel - Re: [patch V4 part 1 20/36] vmlinux.lds.h: Create section for protection against instrumentation

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Date:   Wed, 6 May 2020 09:08:31 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 1 20/36] vmlinux.lds.h: Create section for
 protection against instrumentation

On Tue, May 05, 2020 at 03:16:22PM +0200, Thomas Gleixner wrote:
> Provide also a set of markers: instr_begin()/end()
> 
> These are used to mark code inside a noinstr function which calls
> into regular instrumentable text section as safe.

...

> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -120,10 +120,27 @@ void ftrace_likely_update(struct ftrace_
>  /* Annotate a C jump table to allow objtool to follow the code flow */
>  #define __annotate_jump_table __section(.rodata..c_jump_table)
>  
> +/* Begin/end of an instrumentation safe region */
> +#define instr_begin() ({						\
> +	asm volatile("%c0:\n\t"						\
> +		     ".pushsection .discard.instr_begin\n\t"		\
> +		     ".long %c0b - .\n\t"				\
> +		     ".popsection\n\t" : : "i" (__COUNTER__));		\
> +})
> +
> +#define instr_end() ({							\
> +	asm volatile("%c0:\n\t"						\
> +		     ".pushsection .discard.instr_end\n\t"		\
> +		     ".long %c0b - .\n\t"				\
> +		     ".popsection\n\t" : : "i" (__COUNTER__));		\
> +})

Any chance we could spell these out, i.e. instrumentation_begin/end()?  I
can't help but read these as "instruction_begin/end".  At a glance, the
long names shouldn't cause any wrap/indentation issues.

E.g. some of the usage in KVM is especially confusing

--- a/arch/x86/kvm/vmx/ops.h
+++ b/arch/x86/kvm/vmx/ops.h
@@ -146,7 +146,9 @@ do {						\
 			  : : op1 : "cc" : error, fault);		\
 	return;								\
 error:									\
+	instr_begin();							\
 	insn##_error(error_args);					\
+	instr_end();							\
 	return;								\
 fault:									\
 	kvm_spurious_fault();						\
@@ -161,7 +163,9 @@ do {						\
 			  : : op1, op2 : "cc" : error, fault);		\
 	return;								\
 error:									\
+	instr_begin();							\
 	insn##_error(error_args);					\
+	instr_end();							\
 	return;								\
 fault:									\
 	kvm_spurious_fault();						\