Re: [PATCH v5 3/3] KVM/arm/arm64: enable enhanced armv8 fp/simd lazy switch

[Mario Smarduch <m.smarduch@samsung.com>]

On 12/22/2015 12:06 AM, Christoffer Dall wrote:
> On Mon, Dec 21, 2015 at 11:34:25AM -0800, Mario Smarduch wrote:
>>
>>
>> On 12/18/2015 11:45 PM, Christoffer Dall wrote:
>>> On Fri, Dec 18, 2015 at 05:17:00PM -0800, Mario Smarduch wrote:
>>>> On 12/18/2015 5:54 AM, Christoffer Dall wrote:
>>>>> On Sun, Dec 06, 2015 at 05:07:14PM -0800, Mario Smarduch wrote:
[...]

>>>>>> +  *	we set FPEXC.EN to prevent traps to EL1, when setting the TFP bit.
>>>>>> +  */
>>>>>> +ENTRY(__kvm_vcpu_enable_fpexc32)
>>>>>> +	mov	x3, #(1 << 30)
>>>>>> +	msr	fpexc32_el2, x3
>>>>>> +	isb
>>>>>
>>>>> this is only called via a hypercall so do you really need the ISB?
>>>>
>>>> Same comment as in 2nd patch for the isb.
>>>>
>>>
>>> Unless you can argue that something needs to take effect before
>>> something else, where there's no other implicit barrier, you don't need
>>> the ISB.
>>
>> Make sense an exception level change should be a barrier. It was not there
>> before I put it in due to lack of info on meaning of 'implicit'. The manual has
>> more info on implicit barriers for operations like DMB.
> 
> if the effect from the register write just has to be visible after
> taking an exception, then you don't need the ISB.

Good definition, should be in the manual :)

Thanks.
> 
>>
>> Speaking of ISB it doesn't appear like this one is needed, it's between couple
>> register reads in 'save_time_state' macro.
>>
>> mrc     p15, 0, r2, c14, c3, 1  @ CNTV_CTL
>> str     r2, [vcpu, #VCPU_TIMER_CNTV_CTL]
>>
>> isb
>>
>> mrrc    p15, 3, rr_lo_hi(r2, r3), c14   @ CNTV_CVAL
>>
> 
> I think there was a reason for that one, so let's not worry about that
> for now.
> 
> -Christoffer
>