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Re: [PATCH V4 2/3] xen/arm: Add handling of extended regions for Dom0



Hi Oleksandr,

I saw Stefano committed this patch on Friday. However, I didn't have a chance go to through a second time and would like to request some follow-up changes.

On 30/09/2021 00:52, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

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 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 extended regions could not be allocated for some reason,
Xen doesn't fail and behaves as usual, so only inserts region 0.

Please note the following limitations:
- The extended region feature is only supported for 64-bit domain
   currently.
- The ACPI case is not covered.

***

As Dom0 is direct mapped domain on Arm (e.g. MFN == GFN)
the algorithm to choose extended regions for it is different
in comparison with the algorithm for non-direct mapped DomU.
What is more, that extended regions should be chosen differently
whether IOMMU is enabled or not.

Provide RAM not assigned to Dom0 if IOMMU is disabled or memory
holes found in host device-tree if otherwise. Make sure that
extended regions are 2MB-aligned and located within maximum possible
addressable physical memory range. The minimum size of extended
region is 64MB.

You explained below why the 128 limits, but I don't see any explanation on why 2MB and 64MB.

IIRC, 2MB was to potentally allow superpage mapping. I am not entirely sure for 64MB.

Could you add an in-code comment explaining the two limits?

The maximum number of extended regions is 128,
which is an artificial limit to minimize code changes (we reuse
struct meminfo to describe extended regions, so there are an array
field for 128 elements).

It worth mentioning that unallocated memory solution (when the IOMMU
is disabled) will work safely until Dom0 is able to allocate memory
outside of the original range.

Also introduce command line option to be able to globally enable or
disable support for extended regions for Dom0 (enabled by default).

Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
---
Please note, we need to decide which approach to use in 
find_unallocated_memory(),
you can find details at:
https://lore.kernel.org/xen-devel/28503e09-44c3-f623-bb8d-8778bb94225f@xxxxxxxxx/

Changes RFC -> V2:
    - update patch description
    - drop uneeded "extended-region" DT property

Changes V2 -> V3:
    - update patch description
    - add comment for "size" calculation in add_ext_regions()
    - clarify "end" calculation in find_unallocated_memory() and
      find_memory_holes()
    - only pick up regions with size >= 64MB
    - allocate reg dynamically instead of keeping on the stack in
      make_hypervisor_node()
    - do not show warning for 32-bit domain
    - drop Linux specific limits EXT_REGION_*
    - also cover "ranges" property in find_memory_holes()
    - add command line arg to enable/disable extended region support

Changes V3 -> V4:
   - update opt_ext_regions purpose and comment in code
   - reorganize make_hypervisor_node() to move allocations after initial
     checks, allocate only required amount of elements instead of maximum
     possible
---
  docs/misc/xen-command-line.pandoc |  11 ++
  xen/arch/arm/domain_build.c       | 286 +++++++++++++++++++++++++++++++++++++-

The document in docs/misc/arm/device-tree/guest.txt needs to be updated to reflect the change in the binding.

  2 files changed, 294 insertions(+), 3 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 177e656..5cae4ad 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1081,6 +1081,17 @@ hardware domain is architecture dependent.
  Note that specifying zero as domU value means zero, while for dom0 it means
  to use the default.
+### ext_regions (Arm)
+> `= <boolean>`
+
+> Default : `true`
+
+Flag to enable or disable support for extended regions for Dom0.
+
+Extended regions are ranges of unused address space exposed to Dom0 as
+"safe to use" for special memory mappings. Disable if your board device
+tree is incomplete.
+
  ### flask
  > `= permissive | enforcing | late | disabled`
diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index d233d63..c5afbe2 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -34,6 +34,10 @@
  static unsigned int __initdata opt_dom0_max_vcpus;
  integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
+/* If true, the extended regions support is enabled for dom0 */
+static bool __initdata opt_ext_regions = true;
+boolean_param("ext_regions", opt_ext_regions);
+
  static u64 __initdata dom0_mem;
  static bool __initdata dom0_mem_set;
@@ -886,6 +890,232 @@ static int __init make_memory_node(const struct domain *d,
      return res;
  }
+static int __init add_ext_regions(unsigned long s, unsigned long e, void *data)
+{
+    struct meminfo *ext_regions = data;
+    paddr_t start, size;
+
+    if ( ext_regions->nr_banks >= ARRAY_SIZE(ext_regions->bank) )
+        return 0;
+
+    /* Both start and size of the extended region should be 2MB aligned */
+    start = (s + SZ_2M - 1) & ~(SZ_2M - 1);
+    if ( start > e )
+        return 0;
+
+    /*
+     * e is actually "end-1" because it is called by rangeset functions
+     * which are inclusive of the last address.
+     */
+    e += 1;
+    size = (e - start) & ~(SZ_2M - 1);
+    if ( size < MB(64) )
+        return 0;
+
+    ext_regions->bank[ext_regions->nr_banks].start = start;
+    ext_regions->bank[ext_regions->nr_banks].size = size;
+    ext_regions->nr_banks++;
+
+    return 0;
+}
+
+static int __init find_unallocated_memory(const struct kernel_info *kinfo,
+                                          struct meminfo *ext_regions)

It would be good to have a comment on top of this function summarizing how this is meant to work.

+{
+    const struct meminfo *assign_mem = &kinfo->mem;
+    struct rangeset *unalloc_mem;
+    paddr_t start, end;
+    unsigned int i;
+    int res;
+
+    dt_dprintk("Find unallocated memory for extended regions\n");
+
+    unalloc_mem = rangeset_new(NULL, NULL, 0);
+    if ( !unalloc_mem )
+        return -ENOMEM;
+
+    /* Start with all available RAM */
+    for ( i = 0; i < bootinfo.mem.nr_banks; i++ )
+    {
+        start = bootinfo.mem.bank[i].start;
+        end = bootinfo.mem.bank[i].start + bootinfo.mem.bank[i].size;
+        res = rangeset_add_range(unalloc_mem, start, end - 1);
+        if ( res )
+        {
+            printk(XENLOG_ERR "Failed to add: %#"PRIx64"->%#"PRIx64"\n",

start and end are paddr_t. So you want to use PRIpaddr rather than PRIx64.

+                   start, end);
+            goto out;
+        }
+    }
+
+    /* Remove RAM assigned to Dom0 */
+    for ( i = 0; i < assign_mem->nr_banks; i++ )
+    {
+        start = assign_mem->bank[i].start;
+        end = assign_mem->bank[i].start + assign_mem->bank[i].size;
+        res = rangeset_remove_range(unalloc_mem, start, end - 1);
+        if ( res )
+        {
+            printk(XENLOG_ERR "Failed to remove: %#"PRIx64"->%#"PRIx64"\n",

Ditto.

+                   start, end);
+            goto out;
+        }
+    }
+
+    /* Remove reserved-memory regions */
+    for ( i = 0; i < bootinfo.reserved_mem.nr_banks; i++ )
+    {
+        start = bootinfo.reserved_mem.bank[i].start;
+        end = bootinfo.reserved_mem.bank[i].start +
+            bootinfo.reserved_mem.bank[i].size;
+        res = rangeset_remove_range(unalloc_mem, start, end - 1);
+        if ( res )
+        {
+            printk(XENLOG_ERR "Failed to remove: %#"PRIx64"->%#"PRIx64"\n",

Ditto.

+                   start, end);
+            goto out;
+        }
+    }
+
+    /* Remove grant table region */
+    start = kinfo->gnttab_start;
+    end = kinfo->gnttab_start + kinfo->gnttab_size;
+    res = rangeset_remove_range(unalloc_mem, start, end - 1);
+    if ( res )
+    {
+        printk(XENLOG_ERR "Failed to remove: %#"PRIx64"->%#"PRIx64"\n",

ditto.

+               start, end);
+        goto out;
+    }
+
+    start = 0;
+    end = (1ULL << p2m_ipa_bits) - 1;
+    res = rangeset_report_ranges(unalloc_mem, start, end,
+                                 add_ext_regions, ext_regions);
+    if ( res )
+        ext_regions->nr_banks = 0;
+    else if ( !ext_regions->nr_banks )
+        res = -ENOENT;
+
+out:
+    rangeset_destroy(unalloc_mem);
+
+    return res;
+}
+
+static int __init find_memory_holes(const struct kernel_info *kinfo,
+                                    struct meminfo *ext_regions)

I think it would be good to have a comment on top of the function how this is meant to work.

+{
+    struct dt_device_node *np;
+    struct rangeset *mem_holes;
+    paddr_t start, end;
+    unsigned int i;
+    int res;
+
+    dt_dprintk("Find memory holes for extended regions\n");
+
+    mem_holes = rangeset_new(NULL, NULL, 0);
+    if ( !mem_holes )
+        return -ENOMEM;
+
+    /* Start with maximum possible addressable physical memory range */
+    start = 0;
+    end = (1ULL << p2m_ipa_bits) - 1;
+    res = rangeset_add_range(mem_holes, start, end);
+    if ( res )
+    {
+        printk(XENLOG_ERR "Failed to add: %#"PRIx64"->%#"PRIx64"\n",

Please use PRIpaddr.

+               start, end);
+        goto out;
+    }
+
+    /*
+     * Remove regions described by "reg" and "ranges" properties where
+     * the memory is addressable (MMIO, RAM, PCI BAR, etc).
+     */
+    dt_for_each_device_node( dt_host, np )
+    {
+        unsigned int naddr;
+        u64 addr, size;
+
+        naddr = dt_number_of_address(np);
+
+        for ( i = 0; i < naddr; i++ )
+        {
+            res = dt_device_get_address(np, i, &addr, &size);
+            if ( res )
+            {
+                printk(XENLOG_ERR "Unable to retrieve address %u for %s\n",
+                       i, dt_node_full_name(np));
+                goto out;
+            }
+
+            start = addr & PAGE_MASK;
+            end = PAGE_ALIGN(addr + size);
+            res = rangeset_remove_range(mem_holes, start, end - 1);
+            if ( res )
+            {
+                printk(XENLOG_ERR "Failed to remove: %#"PRIx64"->%#"PRIx64"\n",
+                       start, end);
+                goto out;
+            }
+        }
+
+        if ( dt_device_type_is_equal(np, "pci" ) )
+        {

The code below looks like an open-coding version of dt_for_each_range(). Can you try to re-use it please? This will help to reduce the complexity of this function.

+            unsigned int range_size, nr_ranges;
+            int na, ns, pna;
+            const __be32 *ranges;
+            u32 len;
+
+            /*
+             * Looking for non-empty ranges property which in this context
+             * describes the PCI host bridge aperture.
+             */
+            ranges = dt_get_property(np, "ranges", &len);
+            if ( !ranges || !len )
+                continue;
+
+            pna = dt_n_addr_cells(np);
+            na = dt_child_n_addr_cells(np);
+            ns = dt_child_n_size_cells(np);
+            range_size = pna + na + ns;
+            nr_ranges = len / sizeof(__be32) / range_size;
+
+            for ( i = 0; i < nr_ranges; i++, ranges += range_size )
+            {
+                /* Skip the child address and get the parent (CPU) address */
+                addr = dt_read_number(ranges + na, pna);
+                size = dt_read_number(ranges + na + pna, ns);
+
+                start = addr & PAGE_MASK;
+                end = PAGE_ALIGN(addr + size);
+                res = rangeset_remove_range(mem_holes, start, end - 1);
+                if ( res )
+                {
+                    printk(XENLOG_ERR "Failed to remove: 
%#"PRIx64"->%#"PRIx64"\n",

This should be PRIpaddr.

+                           start, end);
+                    goto out;
+                }
+            }
+        }
+    }
+
+    start = 0;
+    end = (1ULL << p2m_ipa_bits) - 1;
+    res = rangeset_report_ranges(mem_holes, start, end,
+                                 add_ext_regions,  ext_regions);
+    if ( res )
+        ext_regions->nr_banks = 0;
+    else if ( !ext_regions->nr_banks )
+        res = -ENOENT;
+
+out:
+    rangeset_destroy(mem_holes);
+
+    return res;
+}
+
  static int __init make_hypervisor_node(struct domain *d,
                                         const struct kernel_info *kinfo,
                                         int addrcells, int sizecells)
@@ -893,11 +1123,12 @@ static int __init make_hypervisor_node(struct domain *d,
      const char compat[] =
          "xen,xen-"__stringify(XEN_VERSION)"."__stringify(XEN_SUBVERSION)"\0"
          "xen,xen";
-    __be32 reg[4];
+    __be32 *reg, *cells;
      gic_interrupt_t intr;
-    __be32 *cells;
      int res;
      void *fdt = kinfo->fdt;
+    struct meminfo *ext_regions = NULL;
+    unsigned int i, nr_ext_regions;
dt_dprintk("Create hypervisor node\n"); @@ -919,12 +1150,61 @@ static int __init make_hypervisor_node(struct domain *d,
      if ( res )
          return res;
+ if ( !opt_ext_regions )
+    {
+        printk(XENLOG_DEBUG "The extended regions support is disabled\n");

The extended regions is going to be critical for the performance in dom0. So I think this at least want to be a XENLOG_INFO.

+        nr_ext_regions = 0;
+    }
+    else if ( is_32bit_domain(d) )
+    {
+        printk(XENLOG_DEBUG "The extended regions are only supported for 64-bit 
guest currently\n");

This would want to be use XENLOG_WARN.

+        nr_ext_regions = 0;
+    }
+    else
+    {
+        ext_regions = xzalloc(struct meminfo);
+        if ( !ext_regions )
+            return -ENOMEM;
+
+        if ( !is_iommu_enabled(d) )
+            res = find_unallocated_memory(kinfo, ext_regions);
+        else
+            res = find_memory_holes(kinfo, ext_regions);
+
+        if ( res )
+            printk(XENLOG_WARNING "Failed to allocate extended regions\n");
+        nr_ext_regions = ext_regions->nr_banks;
+    }
+
+    reg = xzalloc_array(__be32, (nr_ext_regions + 1) * (addrcells + 
sizecells));
+    if ( !reg )
+    {
+        xfree(ext_regions);
+        return -ENOMEM;
+    }
+
      /* reg 0 is grant table space */
      cells = &reg[0];
      dt_child_set_range(&cells, addrcells, sizecells,
                         kinfo->gnttab_start, kinfo->gnttab_size);
+    /* reg 1...N are extended regions */
+    for ( i = 0; i < nr_ext_regions; i++ )
+    {
+        u64 start = ext_regions->bank[i].start;
+        u64 size = ext_regions->bank[i].size;
+
+        dt_dprintk("Extended region %d: %#"PRIx64"->%#"PRIx64"\n",
+                   i, start, start + size);

I would prefer if this is a printk() so we get the extended region listed from the beginning.

+
+        dt_child_set_range(&cells, addrcells, sizecells, start, size);
+    }
+
      res = fdt_property(fdt, "reg", reg,
-                       dt_cells_to_size(addrcells + sizecells));
+                       dt_cells_to_size(addrcells + sizecells) *
+                       (nr_ext_regions + 1));
+    xfree(ext_regions);
+    xfree(reg);
+
      if ( res )
          return res;

--
Julien Grall



 


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