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

[Julien Grall <julien.grall@xxxxxxx>]

Hello,

On 23/06/16 06:49, Corneliu ZUZU wrote:
> 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;
This is not safe at all, p2m_restore_state is vCPU specific at you 
modify domain information.
However, if we store the hcr per domain, overriding every context switch 
is pointless as the domain will always be 32-bit/64-bit.
> >     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;
> >     }
This does not need to be done in vcpu_enter_adjust_traps everytime. You 
can set the bit in arch.hcr in DOMCTL_MONITOR_EVENT_WRITE_CTRLREG.
> >     update_hcr(v->domain);
> >
> > That way at least it's easier to follow where update_hcr is called.
I don't see much reason to store the value in the domain and have 
multiple update_hcr. If we store the value, then we should only call 
update_hcr once when returning to the guest.
> 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)
We control the value of HCR_EL2, so it would be better to assign the 
list of flags here.
Regards,

--
Julien Grall

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