linux-kernel - Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly

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Date:	Thu, 09 Nov 2006 16:52:40 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	kvm-devel@...ts.sourceforge.net, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly

Arnd Bergmann wrote:
> On Thursday 09 November 2006 14:36, Avi Kivity wrote:
>   
>>> I'm not an expert on inline assembly, but don't you need an extra
>>> '"m" (phys_addr)' to make sure that gcc actually puts the variable
>>> on the stack instead of passing a NULL pointer as '"a"(&phys_addr)'?
>>>       
>> Taking a variable's address should force its contents into memory (like 
>> calling an uninlined function with &var).
>>     
>
> No it doesn't. You're not telling gcc that the inline assembly cares
> about the contents of the variable, so it could be a reference to
> a stack slot while the contents are still in a register. 

Wouldn't that make inline assembly useless?  Suppose the contents is 
itself a pointer.  What about the pointed-to contents?

e.g.

    int x = 3;
    int *y = &x;
    int z;

    asm ("mov %1, %%rax; movl (%%rax), %0" : "=r"(z) : "g"(y) : "rax");
    assert(z == 3);

> Or gcc
> might move the assignment of phys_addr to after the inline assembly.
>   
"asm volatile" prevents that (and I'm not 100% sure it's necessary).


-- 
error compiling committee.c: too many arguments to function

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