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

[Corneliu ZUZU <czuzu@bitdefender.com>]

On 6/22/2016 10:41 PM, Julien Grall wrote:
>
>
> On 22/06/2016 20:37, Corneliu ZUZU wrote:
>> I've also realized that it's a bit complicated to avoid writing HCR from
>> 2 places.
>> That's because:
>> - p2m_restore_state is part of the process of switching to another vCPU
>> and the HCR write _must be committed_ here because other components
>> depend on that, like address-translation functions
>> - I want vm_event_vcpu_enter to be called _after_ the switch to the vCPU
>> is completed
>> - I want HCR_TVM to be set in vm_event_vcpu_enter because setting
>> necessary traps _for cr vm-events_ to work should be done there (setting
>> HCR_TVM bit makes sense to be there and the purpose is to centralize
>> operations such as this for code comprehensibility; also, on the X86
>> counterpart a similar operation is done for trapping CR3, so it would be
>> nice to keep the symmetry)
>>
>> Would it be such a stretch to have HCR written in 2 places? (the second
>> time happens rarely anyway: it's unlikely(..) to have to do the write in
>> vm_event_vcpu_enter)
>
> Not really. It was mostly to avoid setting/clearing HCR bits in 
> different place in the code. It makes more difficult to know what is 
> the final result of the register.
>
> Anyway, let's skip it for now, if it is too difficult.
>
> Regards,
>

Then perhaps something like the following would be suitable:

1. store hcr in arch_domain (register_t hcr)

2. add a function in asm-arm/processor.h (or where else?) which only does:
     static inline void update_hcr(struct domain *d)
     {
         WRITE_SYSREG(d->arch.hcr, HCR_EL2);
         isb();
     }

3.  modify p2m_restore_state to do:
     n->domain->arch.hcr &= ~HCR_VM;
     update_hcr(n->domain);
     p2m_load_VTTBR(n->domain);

     n->domain->arch.hcr |= HCR_VM;

     if ( is_32bit_domain(n->domain) )
         n->domain->arch.hcr &= ~HCR_RW;
     else
         n->domain->arch.hcr |= HCR_RW;

     update_hcr(n->domain);

     WRITE_SYSREG(n->arch.sctlr, SCTLR_EL1);
     isb();

4. and vcpu_enter_adjust_traps to

     if ( unlikely(0 != v->domain->arch.monitor.write_ctrlreg_enabled) )
     {
          if ( likely(v->domain->arch.hcr & HCR_TVM) )
              return;
          v->domain->arch.hcr |= HCR_TVM;
     }
     else
     {
          if ( likely(!(v->domain->arch.hcr & HCR_TVM)) )
              return;
          v->domain->arch.hcr &= ~HCR_TVM;
     }

     update_hcr(v->domain);

That way at least it's easier to follow where update_hcr is called.

Corneliu.

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