Re: [Xen-devel] [PATCH 02/18] xen/arm: Restore HCR_EL2 register

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 15/03/17 08:34, Wei Chen wrote:
> On 2017/3/15 8:25, Stefano Stabellini wrote:
> > On Mon, 13 Mar 2017, Wei Chen wrote:
> > > Different domains may have different HCR_EL2 flags. For example, the
> > > 64-bit domain needs HCR_RW flag but the 32-bit does not need it. So
> > > we give each domain a default HCR_EL2 value and save it in the VCPU's
> > > context.
> > >
> > > HCR_EL2 register has only one bit can be updated automatically without
> > > explicit write (HCR_VSE). But we haven't used this bit currently, so
> > > we can consider that the HCR_EL2 register will not be modified while
> > > the guest is running. So save the HCR_EL2 while guest exiting to
> > > hypervisor is not neccessary. We just have to restore this register for
> > > each VCPU while leaving hypervisor.
> > >
> > > We prefer to restore HCR_EL2 in leave_hypervisor_tail rather than in
> > > ctxt_switch_to. Because the leave_hypervisor_tail is the closest place
> > > to the exception return. In this case, we don't need to warrant the
> > > HCR_EL2 will not be changed between ctxt_switch_to and exception return.
> > Please explain a bit better why it is good to restore HCR_EL2 in
> > leave_hypervisor_tail. Why is it a good thing that is closer to the
> > exception return? What can be the cause of a change in HCR_EL2?
> Ok, I will try to improve it in next version. In current Xen code, I
> can't see any code would change the HCR_EL2 between ctxt_switch_to and
> return_from_trap. But my concern is that, in the future, if someone
> want to insert some HCR_EL2 change code between them, we can't guarantee
> he will restore correct HCR_EL2 value for current vcpu.
Well, the purpose of this series is to inject a virtual SError to the 
guest when a host SError is happening. The host SError will be received 
in the hypervisor whilst the vCPU is running and no context switch (e.g 
call to ctxt_switch) may happen if the scheduler decides to keep the 
vCPU running.
Also, one could argue that HCR_EL2 could be modified on-fly in the 
function but we may have other places in the future which will modify 
HCR_EL2. For instance, this would be the case when the monitor app will 
gain support of inspecting some registers.
So we want a single place to restore HCR_EL2. And leave_hypervisor_tail 
is the best place for that.
Cheers,

--
Julien Grall

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