Re: [PATCH V2 4/4] arm/xen: Read extended regions from DT and init Xen resource

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 26 Oct 2021, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> This patch implements arch_xen_unpopulated_init() on Arm where
> the extended regions (if any) are gathered from DT and inserted
> into passed Xen resource to be used as unused address space
> for Xen scratch pages by unpopulated-alloc code.
> 
> The extended region (safe range) is a region of guest physical
> address space which is unused and could be safely used to create
> grant/foreign mappings instead of wasting real RAM pages from
> the domain memory for establishing these mappings.
> 
> The extended regions are chosen by the hypervisor at the domain
> creation time and advertised to it via "reg" property under
> hypervisor node in the guest device-tree. As region 0 is reserved
> for grant table space (always present), the indexes for extended
> regions are 1...N.
> 
> If arch_xen_unpopulated_init() fails for some reason the default
> behaviour will be restored (allocate xenballooned pages).
> 
> This patch also removes XEN_UNPOPULATED_ALLOC dependency on x86.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
> Changes RFC -> V2:
>    - new patch, instead of
>     "[RFC PATCH 2/2] xen/unpopulated-alloc: Query hypervisor to provide 
> unallocated space"
> ---
>  arch/arm/xen/enlighten.c | 112 
> +++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/xen/Kconfig      |   2 +-
>  2 files changed, 113 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index dea46ec..1a1e0d3 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -62,6 +62,7 @@ static __read_mostly unsigned int xen_events_irq;
>  static phys_addr_t xen_grant_frames;
>  
>  #define GRANT_TABLE_INDEX   0
> +#define EXT_REGION_INDEX    1
>  
>  uint32_t xen_start_flags;
>  EXPORT_SYMBOL(xen_start_flags);
> @@ -303,6 +304,117 @@ static void __init xen_acpi_guest_init(void)
>  #endif
>  }
>  
> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> +int arch_xen_unpopulated_init(struct resource *res)
> +{
> +     struct device_node *np;
> +     struct resource *regs, *tmp_res;
> +     uint64_t min_gpaddr = -1, max_gpaddr = 0;
> +     unsigned int i, nr_reg = 0;
> +     struct range mhp_range;
> +     int rc;
> +
> +     if (!xen_domain())
> +             return -ENODEV;
> +
> +     np = of_find_compatible_node(NULL, NULL, "xen,xen");
> +     if (WARN_ON(!np))
> +             return -ENODEV;
> +
> +     /* Skip region 0 which is reserved for grant table space */
> +     while (of_get_address(np, nr_reg + EXT_REGION_INDEX, NULL, NULL))
> +             nr_reg++;
> +     if (!nr_reg) {
> +             pr_err("No extended regions are found\n");
> +             return -EINVAL;
> +     }
> +
> +     regs = kcalloc(nr_reg, sizeof(*regs), GFP_KERNEL);
> +     if (!regs)
> +             return -ENOMEM;
> +
> +     /*
> +      * Create resource from extended regions provided by the hypervisor to 
> be
> +      * used as unused address space for Xen scratch pages.
> +      */
> +     for (i = 0; i < nr_reg; i++) {
> +             rc = of_address_to_resource(np, i + EXT_REGION_INDEX, &regs[i]);
> +             if (rc)
> +                     goto err;
> +
> +             if (max_gpaddr < regs[i].end)
> +                     max_gpaddr = regs[i].end;
> +             if (min_gpaddr > regs[i].start)
> +                     min_gpaddr = regs[i].start;
> +     }
> +
> +     /* Check whether the resource range is within the hotpluggable range */
> +     mhp_range = mhp_get_pluggable_range(true);
> +     if (min_gpaddr < mhp_range.start)
> +             min_gpaddr = mhp_range.start;
> +     if (max_gpaddr > mhp_range.end)
> +             max_gpaddr = mhp_range.end;
> +
> +     res->start = min_gpaddr;
> +     res->end = max_gpaddr;
> +
> +     /*
> +      * Mark holes between extended regions as unavailable. The rest of that
> +      * address space will be available for the allocation.
> +      */
> +     for (i = 1; i < nr_reg; i++) {
> +             resource_size_t start, end;
> +
> +             start = regs[i - 1].end + 1;
> +             end = regs[i].start - 1;
> +
> +             if (start > (end + 1)) {

Should this be:

if (start >= end)

?


> +                     rc = -EINVAL;
> +                     goto err;
> +             }
> +
> +             /* There is no hole between regions */
> +             if (start == (end + 1))

Also here, shouldn't it be:

if (start == end)

?

I think I am missing again something in termination accounting :-)


> +                     continue;
> +
> +             /* Check whether the hole range is within the resource range */
> +             if (start < res->start || end > res->end) {

By definition I don't think this check is necessary as either condition
is impossible?


> +                     if (start < res->start)
> +                             start = res->start;
> +                     if (end > res->end)
> +                             end = res->end;
> +
> +                     if (start >= (end + 1))
> +                             continue;
> +             }
> +
> +             tmp_res = kzalloc(sizeof(*tmp_res), GFP_KERNEL);
> +             if (!tmp_res) {
> +                     rc = -ENOMEM;
> +                     goto err;
> +             }
> +
> +             tmp_res->name = "Unavailable space";
> +             tmp_res->start = start;
> +             tmp_res->end = end;

Do we need to set any flags so that the system can reuse the memory in
the hole, e.g. IORESOURCE_MEM? Or is it not necessary?


> +             rc = insert_resource(res, tmp_res);
> +             if (rc) {
> +                     pr_err("Cannot insert resource [%llx - %llx] %d\n",
> +                                     tmp_res->start, tmp_res->end, rc);

Although it is impossible to enable XEN_UNPOPULATED_ALLOC on arm32 due
to unmet dependencies, I would like to keep the implementation of
arch_xen_unpopulated_init 32bit clean.

I am getting build errors like (by forcing arch_xen_unpopulated_init to
compile on arm32):

./include/linux/kern_levels.h:5:18: warning: format ‘%llx’ expects argument of 
type ‘long long unsigned int’, but argument 3 has type ‘resource_size_t {aka 
unsigned int}’ [-Wformat=]


> +                     kfree(tmp_res);
> +                     goto err;
> +             }
> +     }
> +
> +err:
> +     kfree(regs);
> +
> +     return rc;
> +}
> +#endif
> +
>  static void __init xen_dt_guest_init(void)
>  {
>       struct device_node *xen_node;
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 1b2c3ac..e6031fc 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -297,7 +297,7 @@ config XEN_FRONT_PGDIR_SHBUF
>  
>  config XEN_UNPOPULATED_ALLOC
>       bool "Use unpopulated memory ranges for guest mappings"
> -     depends on X86 && ZONE_DEVICE
> +     depends on ZONE_DEVICE
>       default XEN_BACKEND || XEN_GNTDEV || XEN_DOM0
>       help
>         Use unpopulated memory ranges in order to create mappings for guest