Re: [PATCH] xen/arm: p2m: Populate pages for GICv2 mapping in arch_domain_create()

[Julien Grall <julien@xxxxxxx>]

On 13/10/2022 10:40, Henry Wang wrote:
> Hi Andrew,
>
> > -----Original Message-----
> > From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
> > Subject: Re: [PATCH] xen/arm: p2m: Populate pages for GICv2 mapping in
> > arch_domain_create()
> >
> > On 13/10/2022 09:38, Henry Wang wrote:
> > > Hardware using GICv2 needs to create a P2M mapping of 8KB GICv2 area
> > > when the domain is created. Considering the worst case of page tables
> > > and keep a buffer, populate 16 pages as the default value to the P2M
> > > pages pool in arch_domain_create() at the domain creation stage to
> > > satisfy the GICv2 requirement.
> > >
> > > Fixes: cbea5a1149ca ("xen/arm: Allocate and free P2M pages from the P2M
> > pool")
> > > Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
> > > Signed-off-by: Henry Wang <Henry.Wang@xxxxxxx>
> > > ---
> > > This should also be backported to 4.13, 4.14, 4.15 and 4.16.
> > > ---
> > >   xen/arch/arm/domain.c | 14 ++++++++++++++
> > >   1 file changed, 14 insertions(+)
> > >
> > > diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> > > index 2c84e6dbbb..e40e2bcba1 100644
> > > --- a/xen/arch/arm/domain.c
> > > +++ b/xen/arch/arm/domain.c
> > > @@ -740,6 +740,20 @@ int arch_domain_create(struct domain *d,
> > >           BUG();
> > >       }
> > >
> > > +    spin_lock(&d->arch.paging.lock);
> > > +    /*
> > > +     * Hardware using GICv2 needs to create a P2M mapping of 8KB GICv2
> > area
> > > +     * when the domain is created. Considering the worst case for page
> > > +     * tables and keep a buffer, populate 16 pages to the P2M pages pool
> > here.
> > > +     */
> > > +    if ( (rc = p2m_set_allocation(d, 16, NULL)) != 0 )
> > > +    {
> > > +        p2m_set_allocation(d, 0, NULL);
> > > +        spin_unlock(&d->arch.paging.lock);
> > > +        goto fail;
> > > +    }
> > > +    spin_unlock(&d->arch.paging.lock);
> > Generally, this would be better written as
> >
> > spin_lock();
> > if ( rc = p2m_set_allocation(16) )
> >      p2m_set_allocation(0)
> > spin_unlock();
> >
> > if ( rc )
> >      goto fail;
> >
> > to reduce the number of spin_unlock() calls and make the error paths
> > more clear.  However...
> I think in Arm's arch_domain_create(), all the error handling are the
> same style using:
>
> if ( (rc = <function>) !=0 )
>      goto fail;
>
> and we need to keep them the same?
We don't have too. I agree with Andrew's point. How about the following:

spin_lock()
rc = ...
spin_unlock()

if ( rc )
  goto fail;

Cheers,

--
Julien Grall