Re: [PATCH v4 4/4] xen/arm: Prevent Dom0 to be loaded when using dom0less

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

Hi Luca,

> On 14 Apr 2021, at 10:14, Luca Fancellu <Luca.Fancellu@arm.com> wrote:
> 
> This patch prevents the dom0 to be loaded skipping its
> building and going forward to build domUs when the dom0
> kernel is not found and at least one domU is present.
> 
> Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
> Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

Cheers,
Bertrand

> ---
> v3 changes:
> - Rephrase documentation
> ---
> docs/features/dom0less.pandoc |  7 +++---
> xen/arch/arm/setup.c          | 42 ++++++++++++++++++++++++++++++++---
> 2 files changed, 43 insertions(+), 6 deletions(-)
> 
> diff --git a/docs/features/dom0less.pandoc b/docs/features/dom0less.pandoc
> index d798596cdf..c9edb529e1 100644
> --- a/docs/features/dom0less.pandoc
> +++ b/docs/features/dom0less.pandoc
> @@ -16,9 +16,10 @@ Multiboot specification has been extended to allow for multiple domains
> to be passed to Xen. See docs/misc/arm/device-tree/booting.txt for more
> information about the Multiboot specification and how to use it.
> 
> -Currently, a control domain ("dom0") is still required, but in the
> -future it will become unnecessary when all domains are created
> -directly from Xen. Instead of waiting for the control domain to be fully
> +Currently, a control domain ("dom0") is still required to manage the DomU
> +domains, but the system can start also without dom0 if the Device Tree
> +doesn't specify the dom0 kernel and it declares one or more domUs.
> +Instead of waiting for the control domain (when declared) to be fully
> booted and the Xen tools to become available, domains created by Xen
> this way are started right away in parallel. Hence, their boot time is
> typically much shorter.
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index b405f58996..ecc4f0ae98 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -793,6 +793,38 @@ static void __init setup_mm(void)
> }
> #endif
> 
> +static bool __init is_dom0less_mode(void)
> +{
> +    struct bootmodules *mods = &bootinfo.modules;
> +    struct bootmodule *mod;
> +    unsigned int i;
> +    bool dom0found = false;
> +    bool domUfound = false;
> +
> +    /* Look into the bootmodules */
> +    for ( i = 0 ; i < mods->nr_mods ; i++ )
> +    {
> +        mod = &mods->module[i];
> +        /* Find if dom0 and domU kernels are present */
> +        if ( mod->kind == BOOTMOD_KERNEL )
> +        {
> +            if ( mod->domU == false )
> +            {
> +                dom0found = true;
> +                break;
> +            }
> +            else
> +                domUfound = true;
> +        }
> +    }
> +
> +    /*
> +     * If there is no dom0 kernel but at least one domU, then we are in
> +     * dom0less mode
> +     */
> +    return ( !dom0found && domUfound );
> +}
> +
> size_t __read_mostly dcache_line_bytes;
> 
> /* C entry point for boot CPU */
> @@ -803,7 +835,7 @@ void __init start_xen(unsigned long boot_phys_offset,
>     int cpus, i;
>     const char *cmdline;
>     struct bootmodule *xen_bootmodule;
> -    struct domain *dom0;
> +    struct domain *dom0 = NULL;
>     int rc;
> 
>     dcache_line_bytes = read_dcache_line_bytes();
> @@ -958,7 +990,10 @@ void __init start_xen(unsigned long boot_phys_offset,
>     enable_errata_workarounds();
> 
>     /* Create initial domain 0. */
> -    dom0 = create_dom0();
> +    if ( !is_dom0less_mode() )
> +        dom0 = create_dom0();
> +    else
> +        printk(XENLOG_INFO "Xen dom0less mode detected\n");
> 
>     heap_init_late();
> 
> @@ -976,7 +1011,8 @@ void __init start_xen(unsigned long boot_phys_offset,
>     if ( acpi_disabled )
>         create_domUs();
> 
> -    domain_unpause_by_systemcontroller(dom0);
> +    if ( dom0 )
> +        domain_unpause_by_systemcontroller(dom0);
> 
>     /* Switch on to the dynamically allocated stack for the idle vcpu
>      * since the static one we're running on is about to be freed. */
> -- 
> 2.17.1
>