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

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

On 6/23/2016 8:31 AM, Corneliu ZUZU wrote:
> 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.
And also set the initial value of HCR at the moment of creation, i.e. in 
arch_domain_create as
d->arch.hcr = READ_SYSREG(HCR_EL2)

Corneliu.

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