Re: [Xen-devel] [PATCH v2 04/19] xen/arm: Restore HCR_EL2 register

[Wei Chen <Wei.Chen@xxxxxxx>]

On 2017/3/31 16:39, Julien Grall wrote:
> Hi Wei,
>
> On 03/31/2017 03:10 AM, Wei Chen wrote:
>> Hi Julien and Stefano,
>>
>> On 2017/3/31 6:03, Stefano Stabellini wrote:
>>> On Thu, 30 Mar 2017, Julien Grall wrote:
>>>> Hi Wei,
>>>>
>>>> On 30/03/17 10:13, Wei Chen wrote:
>>>>> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
>>>>> index de59e5f..8af223e 100644
>>>>> --- a/xen/arch/arm/domain_build.c
>>>>> +++ b/xen/arch/arm/domain_build.c
>>>>> @@ -2171,6 +2171,13 @@ int construct_dom0(struct domain *d)
>>>>>          return rc;
>>>>>
>>>>>      /*
>>>>> +     * The HCR_EL2 will temporarily switch to dom0's HCR_EL2 value
>>>>> +     * by p2m_restore_state. We have to save HCR_EL2 to idle vCPU's
>>>>> +     * context for restoring it in later.
>>>>> +     */
>>>>> +    current->arch.hcr_el2 = READ_SYSREG(HCR_EL2);
>>>>
>>>> I don't understand why we care here. idle vCPU will never restore HCR_EL2 
>>>> nor
>>>> return from the hypervisor.
>>>
>>> I don't understand this either
>>>
>>
>> Yes, idle vCPU will never return from hypervisor. But in construct_dom0,
>> it has one chance to restore HCR_EL2.
>> In in construct_dom0 we will call p2m_restore_state twice.
>>      saved_current = current;
>>      p2m_restore_state(v); --->> This is dom0's vCPU0
>>      [...]
>>      set_current(saved_current);
>>      p2m_restore_state(saved_current); --->> this is idle vCPU0
>>
>> In p2m_restore_state, we will write vcpu->arch.hcr_el2 to HCR_EL2. in
>> this case, we will write an unknown value to HCR_EL2. Even though this
>> would not cause any problem from my current testing. But from code
>> scope, I think it would be a drawback.
>
> The p2m_restore_state will do nothing for idle vCPU (see check at the
> beginning of the function). So there are no issue.
>

Oh, yes, you are right. I missed the idle vCPU check at the beginning.
I would remove this code from next version.

> Cheers,
>


-- 
Regards,
Wei Chen

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