Re: [PATCH 2/3] xen/arm, tools: Add a new HVM_PARAM_MAGIC_BASE_PFN key in HVMOP

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 26 Apr 2024, Henry Wang wrote:
> For use cases such as Dom0less PV drivers, a mechanism to communicate
> Dom0less DomU's static data with the runtime control plane (Dom0) is
> needed. Since on Arm HVMOP is already the existing approach to address
> such use cases (for example the allocation of HVM_PARAM_CALLBACK_IRQ),
> add a new HVMOP key HVM_PARAM_MAGIC_BASE_PFN for storing the magic
> page region base PFN. The value will be set at Dom0less DomU
> construction time after Dom0less DomU's magic page region has been
> allocated.
> 
> To keep consistent, also set the value for HVM_PARAM_MAGIC_BASE_PFN
> for libxl guests in alloc_magic_pages().
> 
> Reported-by: Alec Kwapis <alec.kwapis@xxxxxxxxxxxxx>
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
> ---
>  tools/libs/guest/xg_dom_arm.c   | 2 ++
>  xen/arch/arm/dom0less-build.c   | 2 ++
>  xen/arch/arm/hvm.c              | 1 +
>  xen/include/public/hvm/params.h | 1 +
>  4 files changed, 6 insertions(+)
> 
> diff --git a/tools/libs/guest/xg_dom_arm.c b/tools/libs/guest/xg_dom_arm.c
> index 8cc7f27dbb..3c08782d1d 100644
> --- a/tools/libs/guest/xg_dom_arm.c
> +++ b/tools/libs/guest/xg_dom_arm.c
> @@ -74,6 +74,8 @@ static int alloc_magic_pages(struct xc_dom_image *dom)
>      xc_clear_domain_page(dom->xch, dom->guest_domid, base + 
> MEMACCESS_PFN_OFFSET);
>      xc_clear_domain_page(dom->xch, dom->guest_domid, dom->vuart_gfn);
>  
> +    xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_MAGIC_BASE_PFN,
> +            base);
>      xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_CONSOLE_PFN,
>              dom->console_pfn);
>      xc_hvm_param_set(dom->xch, dom->guest_domid, HVM_PARAM_STORE_PFN,
> diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
> index 40dc85c759..72187c167d 100644
> --- a/xen/arch/arm/dom0less-build.c
> +++ b/xen/arch/arm/dom0less-build.c
> @@ -861,6 +861,8 @@ static int __init construct_domU(struct domain *d,
>              free_domheap_pages(magic_pg, 
> get_order_from_pages(NR_MAGIC_PAGES));
>              return rc;
>          }
> +
> +        d->arch.hvm.params[HVM_PARAM_MAGIC_BASE_PFN] = gfn_x(gfn);

I apologize as I have not read the whole email thread in reply to this
patch.

Why do we need to introduce a new hvm param instead of just setting
HVM_PARAM_CONSOLE_PFN and HVM_PARAM_STORE_PFN directly here?