Re: [PATCH v5 3/9] xen/arm: Alloc XenStore page for Dom0less DomUs from hypervisor

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 06/02/2025 02:08, Stefano Stabellini wrote:
> From: Henry Wang <xin.wang2@xxxxxxx>
> 
> There are use cases (for example using the PV driver) in Dom0less
> setup that require Dom0less DomUs start immediately with Dom0, but
> initialize XenStore later after Dom0's successful boot and call to
> the init-dom0less application.
> 
> An error message can seen from the init-dom0less application on
> 1:1 direct-mapped domains:
> ```
> Allocating magic pages
> memory.c:238:d0v0 mfn 0x39000 doesn't belong to d1
> Error on alloc magic pages
> ```
> 
> The "magic page" is a terminology used in the toolstack as reserved
> pages for the VM to have access to virtual platform capabilities.
> Currently the magic pages for Dom0less DomUs are populated by the
> init-dom0less app through populate_physmap(), and populate_physmap()
> automatically assumes gfn == mfn for 1:1 direct mapped domains. This
> cannot be true for the magic pages that are allocated later from the
> init-dom0less application executed in Dom0. For domain using statically
> allocated memory but not 1:1 direct-mapped, similar error "failed to
> retrieve a reserved page" can be seen as the reserved memory list is
> empty at that time.
> 
> Since for init-dom0less, the magic page region is only for XenStore.
> To solve above issue, this commit allocates the XenStore page for
> Dom0less DomUs at the domain construction time. The PFN will be
> noted and communicated to the init-dom0less application executed
> from Dom0. To keep the XenStore late init protocol, set the connection
> status to XENSTORE_RECONNECT.
> 
> Reported-by: Alec Kwapis <alec.kwapis@xxxxxxxxxxxxx>
> Suggested-by: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
> ---
>  xen/arch/arm/dom0less-build.c | 55 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 54 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
> index 49d1f14d65..046439eb87 100644
> --- a/xen/arch/arm/dom0less-build.c
> +++ b/xen/arch/arm/dom0less-build.c
> @@ -1,5 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0-only */
>  #include <xen/device_tree.h>
> +#include <xen/domain_page.h>
>  #include <xen/err.h>
>  #include <xen/event.h>
>  #include <xen/grant_table.h>
> @@ -11,6 +12,8 @@
>  #include <xen/sizes.h>
>  #include <xen/vmap.h>
>  
> +#include <public/io/xs_wire.h>
> +
>  #include <asm/arm64/sve.h>
>  #include <asm/dom0less-build.h>
>  #include <asm/domain_build.h>
> @@ -704,6 +707,53 @@ static int __init alloc_xenstore_evtchn(struct domain *d)
>      return 0;
>  }
>  
> +#define XENSTORE_PFN_OFFSET 1
> +static int __init alloc_xenstore_page(struct domain *d)
> +{
> +    struct page_info *xenstore_pg;
> +    struct xenstore_domain_interface *interface;
> +    mfn_t mfn;
> +    gfn_t gfn;
> +    int rc;
> +
> +    if ( (UINT_MAX - d->max_pages) < 1 )
> +    {
> +        printk(XENLOG_ERR "%pd: Over-allocation for d->max_pages by 1 
> page.\n",
> +               d);
> +        return -EINVAL;
> +    }
empty line here

> +    d->max_pages += 1;
If this patch is separate from modifying init-dom0less, max_pages will be 
bumped twice. Here and in init-dom0less.
Shouldn't we fold it in? The rest is ok.

~Michal