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

[Wei Chen <Wei.Chen@xxxxxxx>]

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.



-- 
Regards,
Wei Chen

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