From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 2/2] kvm-unit-tests: VMX: Test cases for nested EPT
Date: Mon, 09 Sep 2013 16:26:58 +0200
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Cc: kvm <kvm@vger.kernel.org>, Jan Kiszka <jan.kiszka@web.de>,
	Gleb Natapov <gleb@redhat.com>
To: Arthur Chunqi Li <yzt356@gmail.com>
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Il 09/09/2013 16:11, Arthur Chunqi Li ha scritto:
>>> >> +volatile u32 stage;
>>> >> +volatile bool init_fail;
>> >
>> > Why volatile?
> Because init_fail is only set but not used later in ept_init(), and if
> I don't add volatile, compiler may optimize the setting to init_fail.
> 
> This occasion firstly occurred when I write set_stage/get_stage. If
> one variant is set in a function but not used later, the compiler
> usually optimizes this setting as redundant assignment and remove it.

No, the two are different.  "stage" is written several times in the same
function, with no code in the middle:

	stage++;
	*p = 1;
	stage++;

To the compiler, the first store is dead.  The compiler doesn't know
that "*p = 1" traps to the hypervisor.

But this is not the case for "init_fail".

Paolo