Re: KVM 29: Page fault in kernel mode while booting GNU/kFreeBSD

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Aurelien Jarno wrote:
> On Fri, Jul 20, 2007 at 09:57:55AM +0300, Avi Kivity wrote:
>   
>> Avi Kivity wrote:
>>     
>>>> Please tell me what I can do to help debugging this problem. 
>>>> Alternatively I have uploaded an image to reproduce the problem here:
>>>> http://temp.aurel32.net/kfreebsd-amd64.img.g
>>>>     
>>>>         
>>> I've downloaded the image and booted it three times using kvm HEAD. 
>>> Each time I got to the login prompt.
>>>
>>> Is this always reproducible?  Can you check with kvm HEAD (not that I
>>> think of anything that could have fixed this).
>>>
>>>   
>>>       
>> Sorry, that was on Intel.  It reproduces easily on AMD.
>>     
>
> By comparing the cases were the old and the new condition are different,
> I have found that the problem occurs for the lmsw and smsw instructions.
>   

Ah, I was looking at amd-specific instructions (vmx decodes lmsw) but
missed these two.  Good work.

> The patch below is very ugly, but it workarounds the problem.
>   

It's not ugly, it just lives in an ugly function.  But it does lack a
changelog and a signoff.

As to the ugliness, prehaps no_wb should be part of the opcode
descriptor bitmask (opcode_table and friend).

> diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
> index 21ce977..6c1523f 100644
> --- a/drivers/kvm/x86_emulate.c
> +++ b/drivers/kvm/x86_emulate.c
> @@ -1206,11 +1206,13 @@ twobyte_insn:
>  				goto cannot_emulate;
>  			*(u16 *)&_regs[modrm_rm]
>  				= realmode_get_cr(ctxt->vcpu, 0);
> +			no_wb = 1;
>  			break;
>  		case 6: /* lmsw */
>  			if (modrm_mod != 3)
>  				goto cannot_emulate;
>  			realmode_lmsw(ctxt->vcpu, (u16)modrm_val, &_eflags);
> +			no_wb = 1;
>  			break;
>  		case 7: /* invlpg*/
>
>   


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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