Re: [Xen-devel] [PATCH V6 08/10] xen: Add arch_domain_preinit to initialise vGIC before evtchn_init

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Chen,

On 11/06/2015 07:16, Chen Baozi wrote:
> On Thu, Jun 11, 2015 at 10:37:05AM +0100, Ian Campbell wrote:
> > On Thu, 2015-06-11 at 17:20 +0800, Chen Baozi wrote:
> > > On Fri, Jun 05, 2015 at 05:22:56PM +0100, Ian Campbell wrote:
> > > > On Mon, 2015-06-01 at 20:56 +0800, Chen Baozi wrote:
> > > > > From: Chen Baozi <baozich@xxxxxxxxx>
> > > > >
> > > > > evtchn_init will call domain_max_vcpus to allocate poll_mask. On
> > > > > arm/arm64 platform, this number is determined by the vGIC the guest
> > > > > is going to use, which won't be initialised until arch_domain_create
> > > > > is called in current implementation. However, moving arch_domain_create
> > > > > means that we will allocate memory before checking the XSM policy,
> > > > > which seems not to be acceptable because if the domain is not allowed
> > > > > to boot by XSM policy the expensive execution of arch_domain_create
> > > > > is wasteful. Thus, we create the arch_domain_preinit to make vgic_ops
> > > > > initialisation be done earlier.
> > > > I don't have a fundamental objection to this refactoring, but I'm
> > > > curious under what circumstances something would belong in preinit
> > > > rather than create, i.e. what is the expected semantics of this new hook
> > > > vs the old one.
> > > Hmmm, I didn't think about it at this level, :P. Instead, I just brought
> > > the code which must be executed before evtchn_init in advance without
> > > further consideration on semantics etc...
> > >
> > > In fact, the current arch_domain_preinit on arm is all about vgic. I
> > > was about to use the name vgic_preinit, but that would be an arm-specific
> > > name which I thought was not suitable to add in the common codes...
> > >
> > > Or will it be clearer to call a subfunction called vgic_preinit in the
> > > arch_domain_preinit (for arm) and put the current main contents of
> > > arch_domain_preinit to vgic_preinit?
> > The main question which needs answering is: given some new bit of
> > functionality which needs initialising when should it be done in preinit
> > and when should it be in init?
> The resource that would be used (either directly or indirectly) by
> xsm_domain_create, evtchn_init or grant_table_create, should be initialised
> in preinit? otherwise, it should be put into init?
>
> I am not sure, for it doesn't look like a(n) good/exact semantic
> definition...
Rather than splitting arch_domain_init, I was thinking to handle 
domain_max_vcpus differently:
domain_max_vcpus(struct domain *d)
{
   if ( !d->arch.vgic.ops )
     return MAX_VIRT_CPUS;
   else
     return (min(MAX_VIRT_CPUS, d->arch.vgic.ops.max_vcpus));
}

On ARM32, event channel doesn't need to allocate the poll_mask 
(MAX_VIRT_CPUS < BITS_PER_LONG). So no concern.
On ARM64, we have to always allocate the vCPU mask. This wouldn't be so 
bad to allocate more memory (2 unsigned long rather than 1 for GICv2).
Regards,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel