Re: [GIT PULL] Correct the SMAP check in the e820 probe

[Chuck Ebbert <cebbert@redhat.com>]

On 09/28/2007 10:27 AM, Linus Torvalds wrote:
> 
> On Fri, 28 Sep 2007, H. Peter Anvin wrote:
>>     [x86 setup] Correct the SMAP check for INT 0x15, AX=0xe820
>>     
>>     The e820 probe code was checking %edx, not %eax, for the SMAP
>>     signature on return.  This worked on *almost* all systems, since %edx
>>     still contained SMAP from the call on entry, but on a handful of
>>     systems it failed -- plus, we would have missed real mismatches.
>>     
>>     Signed-off-by: H. Peter Anvin <hpa@zytor.com>
>>
>> diff --git a/arch/i386/boot/memory.c b/arch/i386/boot/memory.c
>> index bccaa1c..2f37568 100644
>> --- a/arch/i386/boot/memory.c
>> +++ b/arch/i386/boot/memory.c
>> @@ -28,11 +28,10 @@ static int detect_memory_e820(void)
>>  
>>  	do {
>>  		size = sizeof(struct e820entry);
>> -		id = SMAP;
>>  		asm("int $0x15; setc %0"
>> -		    : "=am" (err), "+b" (next), "+d" (id), "+c" (size),
>> +		    : "=dm" (err), "+b" (next), "=a" (id), "+c" (size),
>>  		      "=m" (*desc)
>> -		    : "D" (desc), "a" (0xe820));
>> +		    : "D" (desc), "d" (SMAP), "a" (0xe820));
> 
> Hmm. If I read this correctly, I don't think this can be right.
> 
> Why? You don't mark %edx as possibly corrupted by the asm any more.
> 
> The "=dm" means that quite often (probably effectively always), gcc will 
> allocate %edx to be the output register for %0, but at least in theory, it 
> could easily decide that it's going to put %0 in memory, and in that case, 
> it may well decide that %edx is not modified by the asm statement. Which 
> may or may not be true - I'd bet that there are BIOSes out there that *do* 
> modify it.
> 
> So what happens then? If gcc decides that %edx isn't modified by the asm, 
> it will assume that it still contains the value it had on entry, which is 
> the "SMAP" value, and then it might decide to do the
> 
> 	if (id != SMAP) {
> 
> check as a 
> 
> 	cmpl %edx,%eax
> 
> since the "id" return is in %eax, and the compiler decides that it may be 
> cheaper to re-use the register that already contains the constant, than to 
> use a (longer) compare instruction with an explicit constant.
> 
> IOW, I think you need to either (a) _force_ gcc to use %edx for the "err" 
> return, avoiding this issue, or (b) mark edx clobbered (which in turn 
> means that you need to remove it from the output constraint for "err"). I 
> suspect (a) is simpler/more straightforward.
> 

Patch with option (a) applied [output 0 changed to: "=d" (err)] tested and
works on the Dell XPS M1330 that was broken by the previous e820 change.

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