Re: [PATCH v2 4/5] xen/arm: Find unallocated spaces for magic pages of direct-mapped domU

From: Michal Orzel <michal.orzel@amd.com>
To: Henry Wang <xin.wang2@amd.com>, <xen-devel@lists.xenproject.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>,
	Bertrand Marquis <bertrand.marquis@arm.com>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	Alec Kwapis <alec.kwapis@medtronic.com>
Subject: Re: [PATCH v2 4/5] xen/arm: Find unallocated spaces for magic pages of direct-mapped domU
Date: Mon, 11 Mar 2024 14:46:38 +0100	[thread overview]
Message-ID: <4821f30b-94a6-4a2a-8c46-81a9d1e288a4@amd.com> (raw)
In-Reply-To: <20240308015435.4044339-5-xin.wang2@amd.com>

Hi Henry,

On 08/03/2024 02:54, Henry Wang wrote:
> For 1:1 direct-mapped dom0less DomUs, the magic pages should not clash
> with any RAM region. To find a proper region for guest magic pages,
> we can reuse the logic of finding domain extended regions.
> 
> Extract the logic of finding domain extended regions to a helper
> function named find_unused_memory() and use it to find unallocated
> spaces for magic pages before make_hypervisor_node(). The result magic
> page region is added to the reserved memory section of the bootinfo so
> that it is carved out from the extended regions.
> 
> Reported-by: Alec Kwapis <alec.kwapis@medtronic.com>
> Signed-off-by: Henry Wang <xin.wang2@amd.com>
> ---
> v2:
> - New patch
> ---
>  xen/arch/arm/dom0less-build.c           | 43 +++++++++++++++++++++++++
>  xen/arch/arm/domain_build.c             | 30 ++++++++++-------
>  xen/arch/arm/include/asm/domain_build.h |  2 ++
>  3 files changed, 64 insertions(+), 11 deletions(-)
> 
> diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
> index 1e1c8d83ae..99447bfb0c 100644
> --- a/xen/arch/arm/dom0less-build.c
> +++ b/xen/arch/arm/dom0less-build.c
> @@ -682,6 +682,49 @@ static int __init prepare_dtb_domU(struct domain *d, struct kernel_info *kinfo)
>  
>      if ( kinfo->dom0less_feature & DOM0LESS_ENHANCED_NO_XS )
>      {
> +        if ( is_domain_direct_mapped(d) )
> +        {
This whole block is dependent on static memory feature that is compiled out by default.
Shouldn't you move it to static-memory.c ?

> +            struct meminfo *avail_magic_regions = xzalloc(struct meminfo);
I can't see corresponding xfree(avail_magic_regions). It's not going to be used after unused memory
regions are retrieved.

> +            struct meminfo *rsrv_mem = &bootinfo.reserved_mem;
> +            struct mem_map_domain *mem_map = &d->arch.mem_map;
> +            uint64_t magic_region_start = INVALID_PADDR;
What's the purpose of this initialization? magic_region_start is going to be re-assigned before making use of this value.

> +            uint64_t magic_region_size = GUEST_MAGIC_SIZE;
Why not paddr_t?

> +            unsigned int i;
> +
> +            if ( !avail_magic_regions )
> +                return -ENOMEM;
What about memory allocated for kinfo->fdt? You should goto err;

> +
> +            ret = find_unused_memory(d, kinfo, avail_magic_regions);
> +            if ( ret )
> +            {
> +                printk(XENLOG_WARNING
> +                       "%pd: failed to find a region for domain magic pages\n",
> +                      d);
> +                goto err;
What about memory allocated for avail_magic_regions? You should free it.

> +            }
> +
> +            magic_region_start = avail_magic_regions->bank[0].start;
> +
> +            /*
> +             * Register the magic region as reserved mem to make sure this
> +             * region will not be counted when allocating extended regions.
Well, this is only true in case find_unallocated_memory() is used to retrieve free regions.
What if our direct mapped domU used partial dtb and IOMMU is in use? In this case,
find_memory_holes() will be used and the behavior will be different.

Also, I'm not sure if it is a good idea to call find_unused_memory twice (with lots of steps inside)
just to retrieve 16MB (btw. add_ext_regions will only return 64MB+ regions) region for magic pages.
I'll let other maintainers share their opinion.

Also, CCing Carlo since he was in a need of retrieving free memory regions as well for cache coloring with dom0.

~Michal