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

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

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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