Re: [PATCH 7/7] vm-event/arm: implement support for control-register write vm-events

[Julien Grall <julien.grall@arm.com>]

Hello,

On 22/06/16 18:17, Julien Grall wrote:
> On 22/06/16 17:35, Corneliu ZUZU wrote:
>
>> Julien,
>
> Hello Corneliu,
>
>> I was trying to implement having HCR stored in arch_domain or arch_vcpu
>> as suggested above and I'm a bit confused about the code in
>> p2m_restore_state.
>> I'm hoping you can provide some feedback on this matter. Here's the
>> current implementation of the function:
>>
>>      void p2m_restore_state(struct vcpu *n)
>>      {
>>          register_t hcr;
>>
>>          hcr = READ_SYSREG(HCR_EL2);
>>          WRITE_SYSREG(hcr & ~HCR_VM, HCR_EL2);
>>          isb();
>>
>>          p2m_load_VTTBR(n->domain);
>>          isb();
>>
>>          if ( is_32bit_domain(n->domain) )
>>              hcr &= ~HCR_RW;
>>          else
>>              hcr |= HCR_RW;
>>
>>          WRITE_SYSREG(n->arch.sctlr, SCTLR_EL1);
>>          isb();
>>
>>          WRITE_SYSREG(hcr, HCR_EL2);
>>          isb();
>>      }
>>
>> First of all, I see the HCR_VM bit being unset (=0) but I don't quite
>> understand why and even more peculiar is the fact that I couldn't find
>> any place where the bit is set (=1) again.
>
> After the first write to HCR_EL2, "hcr" still have the VM bit set as we
> only mask it. So the second write will re-set the VM bit.
>
> I am not sure why the VM bit is unset/set in this function. I am not
> able to find a paragraph justifying it in the ARM ARM. I have CCed some
> ARM folks to check if I missed something.

After talking with different ARM folks, I confirm that it is not 
necessary to disable the HCR_VM before setting TTBR0.

Regards,

-- 
Julien Grall

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