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

[Jan Beulich <jbeulich@xxxxxxxx>]

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(). 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?

Jan