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

From: Stefano Stabellini
Date: Wed Oct 27 2021 - 21:40:13 EST


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