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Re: [RFC PATCH 2/2] xen/arm: Rework dt_unreserved_regions to avoid recursion
Hi Luca,
On 25/04/2024 14:11, Luca Fancellu wrote:
The function dt_unreserved_regions is currently using recursion
to compute the non overlapping ranges of a passed region against
the reserved memory banks, in the spirit of removing the recursion
to improve safety and also improve the scalability of the function,
rework its code to use an iterative algorithm with the same result.
The function was taking an additional parameter 'first', but given
the rework and given that the function was always initially called
with this parameter as zero, remove the parameter and update the
codebase to reflect the change.
In general, I am in favor to remove the recursion. I have some
questions/remarks below though.
Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
---
xen/arch/arm/include/asm/setup.h | 8 +-
xen/arch/arm/include/asm/static-shmem.h | 7 +-
xen/arch/arm/kernel.c | 2 +-
xen/arch/arm/setup.c | 133 ++++++++++++++++--------
4 files changed, 96 insertions(+), 54 deletions(-)
diff --git a/xen/arch/arm/include/asm/setup.h b/xen/arch/arm/include/asm/setup.h
index fc6967f9a435..24519b9ed969 100644
--- a/xen/arch/arm/include/asm/setup.h
+++ b/xen/arch/arm/include/asm/setup.h
@@ -9,7 +9,12 @@
#define MAX_FDT_SIZE SZ_2M
#define NR_MEM_BANKS 256
+
+#ifdef CONFIG_STATIC_SHM
#define NR_SHMEM_BANKS 32
+#else
+#define NR_SHMEM_BANKS 0
+#endif
#define MAX_MODULES 32 /* Current maximum useful modules */
@@ -215,8 +220,7 @@ void create_dom0(void);
void discard_initial_modules(void);
void fw_unreserved_regions(paddr_t s, paddr_t e,
- void (*cb)(paddr_t ps, paddr_t pe),
- unsigned int first);
+ void (*cb)(paddr_t ps, paddr_t pe));
size_t boot_fdt_info(const void *fdt, paddr_t paddr);
const char *boot_fdt_cmdline(const void *fdt);
diff --git a/xen/arch/arm/include/asm/static-shmem.h
b/xen/arch/arm/include/asm/static-shmem.h
index 3b6569e5703f..1b7c7ea0e17d 100644
--- a/xen/arch/arm/include/asm/static-shmem.h
+++ b/xen/arch/arm/include/asm/static-shmem.h
@@ -7,11 +7,11 @@
#include <asm/kernel.h>
#include <asm/setup.h>
-#ifdef CONFIG_STATIC_SHM
-
/* Worst case /memory node reg element: (addrcells + sizecells) */
#define DT_MEM_NODE_REG_RANGE_SIZE ((NR_MEM_BANKS + NR_SHMEM_BANKS) * 4)
+#ifdef CONFIG_STATIC_SHM
+
int make_resv_memory_node(const struct kernel_info *kinfo, int addrcells,
int sizecells);
@@ -47,9 +47,6 @@ void shm_mem_node_fill_reg_range(const struct kernel_info *kinfo, __be32 *reg,
#else /* !CONFIG_STATIC_SHM */
-/* Worst case /memory node reg element: (addrcells + sizecells) */
-#define DT_MEM_NODE_REG_RANGE_SIZE (NR_MEM_BANKS * 4)
-
I don't understand how this is related to the rest patch. Can you clarify?
static inline int make_resv_memory_node(const struct kernel_info *kinfo,
int addrcells, int sizecells)
{
diff --git a/xen/arch/arm/kernel.c b/xen/arch/arm/kernel.c
index 674388fa11a2..ecbeec518754 100644
--- a/xen/arch/arm/kernel.c
+++ b/xen/arch/arm/kernel.c
@@ -247,7 +247,7 @@ static __init int kernel_decompress(struct bootmodule *mod,
uint32_t offset)
* Free the original kernel, update the pointers to the
* decompressed kernel
*/
- fw_unreserved_regions(addr, addr + size, init_domheap_pages, 0);
+ fw_unreserved_regions(addr, addr + size, init_domheap_pages);
return 0;
}
diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index c4e5c19b11d6..d737fe56e539 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -204,55 +204,97 @@ static void __init processor_id(void)
}
static void __init dt_unreserved_regions(paddr_t s, paddr_t e,
- void (*cb)(paddr_t ps, paddr_t pe),
- unsigned int first)
+ void (*cb)(paddr_t ps, paddr_t pe))
{
- const struct membanks *reserved_mem = bootinfo_get_reserved_mem();
-#ifdef CONFIG_STATIC_SHM
- const struct membanks *shmem = bootinfo_get_shmem();
- unsigned int offset;
-#endif
- unsigned int i;
-
/*
- * i is the current bootmodule we are evaluating across all possible
- * kinds.
+ * The worst case scenario is to have N reserved region ovelapping the
+ * passed one, so having N+1 regions in the stack
*/
- for ( i = first; i < reserved_mem->nr_banks; i++ )
- {
- paddr_t r_s = reserved_mem->bank[i].start;
- paddr_t r_e = r_s + reserved_mem->bank[i].size;
-
- if ( s < r_e && r_s < e )
- {
- dt_unreserved_regions(r_e, e, cb, i + 1);
- dt_unreserved_regions(s, r_s, cb, i + 1);
- return;
- }
- }
-
+ struct {
+ paddr_t s;
+ paddr_t e;
+ unsigned int dict;
+ unsigned int bank;
+ } stack[NR_MEM_BANKS + NR_SHMEM_BANKS + 1];
If I am not mistaken, the variable could take up to 7KB of the stack.
This is quite a bit (Xen stack is 32KB on Arm).
AFAIK, the function is not expected to be called concurrently. So it
would be best to move to .initdata (by adding static __inidata).
Even with that in mind, do we actually need such a large stack?
+ const struct membanks *mem_banks[] = {
+ bootinfo_get_reserved_mem(),
#ifdef CONFIG_STATIC_SHM
- /*
- * When retrieving the corresponding shared memory addresses
- * below, we need to index the shmem->bank starting from 0, hence
- * we need to use i - reserved_mem->nr_banks.
- */
- offset = reserved_mem->nr_banks;
- for ( ; i - offset < shmem->nr_banks; i++ )
+ bootinfo_get_shmem(),
+#endif
+ };
+ unsigned int count = 0;
+ unsigned int i, j;
i and j only seemed to be used within the loop. So can this be only be
declared within it?
+
+ /* Push the initial region into the stack */
+ stack[count].s = s;
+ stack[count].e = e;
+ stack[count].dict = 0;
+ stack[count].bank = 0;
+ count++;
You are repeating the 5 lines quite a bit. Can we add a wrapper for it?
This would make the code a bit easier to read.
+ while ( count > 0 )
{
- paddr_t r_s = shmem->bank[i - offset].start;
- paddr_t r_e = r_s + shmem->bank[i - offset].size;
-
- if ( s < r_e && r_s < e )
+ paddr_t b_s, b_e;
Coding style: Please add a newline.
+ /* Take out last element of the stack */
+ count--;
+ i = stack[count].dict;
+ j = stack[count].bank;
+ b_s = stack[count].s;
+ b_e = stack[count].e;
+ /* Start checking for overlapping banks */
+ for ( ; i < ARRAY_SIZE(mem_banks); i++ )
{
- dt_unreserved_regions(r_e, e, cb, i + 1);
- dt_unreserved_regions(s, r_s, cb, i + 1);
- return;
+ for ( ; j < mem_banks[i]->nr_banks; j++ )
+ {
+ paddr_t r_s = mem_banks[i]->bank[j].start;
+ paddr_t r_e = r_s + mem_banks[i]->bank[j].size;
As you are reworking the code, I would like us to avoid using 'start +
size' because it could overflow.
I would prefer if we use either start, size or using 'start + size - 1'.
Also coding style: Newlinen missing
+
+ if ( b_s < r_e && r_s < b_e )
+ {
+ /*
The line seems to be misindented compare to the other. Or maybe the rest is?
+ * Region is overlapping, push the two non overlap ranges in
+ * the stack
+ */
+ if ( (count + 2) < ARRAY_SIZE(stack) )
+ {
+ /*
+ * Push initial non overlapping part of the region into
+ * the stack
+ */
+ stack[count].s = b_s;
+ stack[count].e = r_s;
+ stack[count].dict = i;
+ stack[count].bank = j + 1;
+ count++;
+ /*
+ * Push last non overlapping part of the region into
+ * the stack
+ */
+ stack[count].s = r_e;
+ stack[count].e = b_e;
+ stack[count].dict = i;
+ stack[count].bank = j + 1;
+ count++;
+ }
+ else
+ panic("dt_unreserved_regions: stack space exausted");
Can you document in which circumstances this can happen?
+
+ /*
+ * Retrieve the last region pushed to check against other
+ * banks, so break the loop through the banks and pop
+ * a new region from the stack
+ */
+ goto regions_pushed;
+ }
+ }
+ j = 0;
}
+ /*
+ * This point is reached when the region doesn't overlap with any other
+ * region, so call the callback on it.
+ */
+ cb(b_s, b_e);
+ regions_pushed:
}
-#endif
-
- cb(s, e);
}
/*
@@ -316,11 +358,10 @@ static bool __init bootmodules_overlap_check(struct
bootmodules *bootmodules,
}
void __init fw_unreserved_regions(paddr_t s, paddr_t e,
- void (*cb)(paddr_t ps, paddr_t pe),
- unsigned int first)
+ void (*cb)(paddr_t ps, paddr_t pe))
{
if ( acpi_disabled )
- dt_unreserved_regions(s, e, cb, first);
+ dt_unreserved_regions(s, e, cb);
else
cb(s, e);
}
@@ -527,7 +568,7 @@ void __init discard_initial_modules(void)
!mfn_valid(maddr_to_mfn(e)) )
continue;
- fw_unreserved_regions(s, e, init_domheap_pages, 0);
+ fw_unreserved_regions(s, e, init_domheap_pages);
}
mi->nr_mods = 0;
@@ -702,7 +743,7 @@ void __init populate_boot_allocator(void)
}
#endif
- fw_unreserved_regions(s, e, init_boot_pages, 0);
+ fw_unreserved_regions(s, e, init_boot_pages);
s = n;
}
}
Cheers,
--
Julien Grall
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