Re: KVM: Fix the invlpg instruction emulation on AMD64

[Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>]

Avi Kivity a écrit :
> Aurelien Jarno wrote:
>> The patch below removes the check for c->modrm_reg == 7 to detect the 
>> invlpg instruction, as it was the case before before commit 
>> aa38840d3d2e0a804e628077df8d8879b496d741. This fixes the boot of FreeBSD
>> on an AMD64 CPU.
>>
>> It also moves the assignation of c->src.bytes after the test as it is
>> not needed for the invlpg instruction.
>>
>> Signed-off-by: Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>
>>
>> diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
>> index fa33fcd..01aa952 100644
>> --- a/drivers/kvm/x86_emulate.c
>> +++ b/drivers/kvm/x86_emulate.c
>> @@ -824,12 +824,10 @@ modrm_done:
>>  		c->src.bytes = 4;
>>  		goto srcmem_common;
>>  	case SrcMem:
>> -		c->src.bytes = (c->d & ByteOp) ? 1 :
>> -							   c->op_bytes;
>>  		/* Don't fetch the address for invlpg: it could be unmapped. */
>> -		if (c->twobyte && c->b == 0x01
>> -				    && c->modrm_reg == 7)
>> +		if (c->twobyte && c->b == 0x01)
>>  			break;
>> +		c->src.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
>>
>>   
> 
> I don't understand why this helps.  All of the other instructions in 
> this group either have modrm_mod == 3 or do require evaluation of the 
                         ^^^^^^^^^
The test actually concerns modrm_reg and not modrm_mod. Maybe it is wrong?

> source.  invlpg is the only one that doesn't.

I have marked the invlpg instruction the same way as it is done in
kvm-37 to know what happens. I get either modrm_reg = 4 or = 6 when the
invlpg instruction is executed, but never = 7.

> It looks like this patch would break lgdt and lidt (which are only 
> needed on Intel).
> 

I admit I haven't tried on Intel.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32-8fiUuRrzOP0dnm+yROfE0A@public.gmane.org         | aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org
   `-    people.debian.org/~aurel32 | www.aurel32.net

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