linux-kernel - Re: [PATCH v12 03/18] xen/pvh: Early bootup changes in PV code (v2).

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Date:	Fri, 3 Jan 2014 11:29:13 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Mukesh Rathor <mukesh.rathor@...cle.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH v12 03/18] xen/pvh: Early bootup changes in PV code (v2).

On 03/01/14 01:34, Mukesh Rathor wrote:
> On Thu, 2 Jan 2014 13:32:21 -0500
> Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> 
>> On Thu, Jan 02, 2014 at 03:32:33PM +0000, David Vrabel wrote:
>>> On 01/01/14 04:35, Konrad Rzeszutek Wilk wrote:
>>>> From: Mukesh Rathor <mukesh.rathor@...cle.com>
>>>>
>>>> In the bootup code for PVH we can trap cpuid via vmexit, so don't
>>>> need to use emulated prefix call. We also check for vector
>>>> callback early on, as it is a required feature. PVH also runs at
>>>> default kernel IOPL.
>>>>
>>>> Finally, pure PV settings are moved to a separate function that
>>>> are only called for pure PV, ie, pv with pvmmu. They are also
>>>> #ifdef with CONFIG_XEN_PVMMU.
>>> [...]
>>>> @@ -331,12 +333,15 @@ static void xen_cpuid(unsigned int *ax,
>>>> unsigned int *bx, break;
>>>>  	}
>>>>  
>>>> -	asm(XEN_EMULATE_PREFIX "cpuid"
>>>> -		: "=a" (*ax),
>>>> -		  "=b" (*bx),
>>>> -		  "=c" (*cx),
>>>> -		  "=d" (*dx)
>>>> -		: "0" (*ax), "2" (*cx));
>>>> +	if (xen_pvh_domain())
>>>> +		native_cpuid(ax, bx, cx, dx);
>>>> +	else
>>>> +		asm(XEN_EMULATE_PREFIX "cpuid"
>>>> +			: "=a" (*ax),
>>>> +			"=b" (*bx),
>>>> +			"=c" (*cx),
>>>> +			"=d" (*dx)
>>>> +			: "0" (*ax), "2" (*cx));
>>>
>>> For this one off cpuid call it seems preferrable to me to use the
>>> emulate prefix rather than diverge from PV.
>>
>> This was before the PV cpuid was deemed OK to be used on PVH.
>> Will rip this out to use the same version.
> 
> Whats wrong with using native cpuid? That is one of the benefits that
> cpuid can be trapped via vmexit, and also there is talk of making PV
> cpuid trap obsolete in the future. I suggest leaving it native.

It should either use the PV interface or the HVM one, not a hybrid of
the two.

David
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