RE: [PATCH v2] xen/arm: p2m: Populate pages for GICv2 mapping in arch_domain_create()

[Henry Wang <Henry.Wang@xxxxxxx>]

Hi Jan,

Thanks for the review.

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Subject: Re: [PATCH v2] xen/arm: p2m: Populate pages for GICv2 mapping in
> arch_domain_create()
> 
> On 14.10.2022 10:09, Henry Wang wrote:
> > --- a/xen/arch/arm/domain.c
> > +++ b/xen/arch/arm/domain.c
> > @@ -740,6 +740,20 @@ int arch_domain_create(struct domain *d,
> >          BUG();
> >      }
> >
> > +    /*
> > +     * 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.
> > +     * For GICv3, the above-mentioned P2M mapping is not necessary, but
> since
> > +     * the allocated 16 pages here would not be lost, hence populate these
> > +     * pages unconditionally.
> > +     */
> > +    spin_lock(&d->arch.paging.lock);
> > +    rc = p2m_set_allocation(d, 16, NULL);
> > +    spin_unlock(&d->arch.paging.lock);
> > +    if ( rc != 0 )
> > +        goto fail;
> 
> Putting this level of knowledge here feels like a layering violation to
> me. My first suggestion would be to move this call somewhere under
> p2m_init().

That is definitely possible. If Julien or other Arm maintainers are not
against that I am happy to move this to p2m_init() in v3.

The reason why the above block is placed here is just I thought to use
d->arch.vgic.version to only populate the 16 pages for GICv2 in the
beginning, and d->arch.vgic.version is first assigned later after p2m_init(),
but later we decided to populated the pages unconditionally so actually
now we can move the part to p2m_init().

> If that's not possible for some reason, I'd like to suggest
> passing 1 here as the count and then adding a min-acceptable check to
> p2m_set_allocation() along the lines of x86'es shadow_set_allocation().
> That way you'd also guarantee the minimum number of pages in case a
> subsequent tiny allocation request came in via domctl.
> 
> > @@ -1736,6 +1736,17 @@ void p2m_final_teardown(struct domain *d)
> >      if ( !p2m->domain )
> >          return;
> >
> > +    if ( !page_list_empty(&p2m->pages) )
> > +        p2m_teardown(d, false);
> > +
> > +    if ( d->arch.paging.p2m_total_pages != 0 )
> > +    {
> > +        spin_lock(&d->arch.paging.lock);
> > +        p2m_set_allocation(d, 0, NULL);
> > +        spin_unlock(&d->arch.paging.lock);
> > +        ASSERT(d->arch.paging.p2m_total_pages == 0);
> > +    }
> 
> Is it intentional to largely open-code p2m_teardown_allocation() here?

Yes, AFAICT p2m_teardown_allocation() is preemptible and we don't want
any preemption here.

Kind regards,
Henry

> 
> Jan