RE: [PATCH 2/2] arm64: mm: support bootparam max_addr

Subject: Re: [PATCH 2/2] arm64: mm: support bootparam max_addr

On Wed, 15 Dec 2021 at 07:56, Peng Fan (OSS) [off-list ref]
wrote:

quoted

From: Peng Fan <peng.fan@nxp.com>

There is a "mem=[x]" boot parameter, but when there is a whole
reserved by secure TEE, the continuous DRAM area is split with two

memblocks.

quoted

For example, DRAM area [0x40000000, 0xffffffff], when TEE uses
[0x50000000, 0x51000000), the memblock will be split into [0x40000000,
0x50000000) and [0x51000000, 0xffffffff].

If pass "mem=1024MB", the actually max addr will be 0x81000000.
However if need the max addr be 0x80000000, mem=1008MB should be

used.

quoted

There also might be multiple other holes that no visible to Linux,
when we wanna to limit the max addr usable by Linux, using
"max_addr=[X]" is much easier than "mem=[X]"

Signed-off-by: Peng Fan <peng.fan@nxp.com>

mem= is a hack already, please don't add another one. Limiting the memory
like this is far too tricky, given that the kernel itself and the initrd could end up
in memory that is excluded, and we have to go and fix things up if that
happens.

We wanna to use the reserved memory with request_mem_region, but with
commit 86588296acbfb1 ("fdt: Properly handle "no-map" field in the memory region ")

request_mem_region will fail, because the reserved memory are now as
kernel memory.

So we use "mem=X" to work around the issue, but "mem=X" is not user friendly
compared with "max_addr=" when there are multiple holes used by others.

Thanks,
Peng.

quoted

---
 arch/arm64/mm/init.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c index

db63cc885771..3364b5e7a7fe 100644

--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -173,6 +173,7 @@ int pfn_is_map_memory(unsigned long pfn)

EXPORT_SYMBOL(pfn_is_map_memory);

 static phys_addr_t memory_limit __ro_after_init = PHYS_ADDR_MAX;
+static phys_addr_t max_addr __ro_after_init = PHYS_ADDR_MAX;

 /*
  * Limit the memory size that was specified via FDT.

@@ -189,6 +190,18 @@ static int __init early_mem(char *p)  }

early_param("mem", early_mem);

+static int __init set_max_addr(char *p) {
+       if (!p)
+               return 1;
+
+       max_addr = memparse(p, &p) & PAGE_MASK;
+       pr_notice("Memory max addr set to 0x%llx\n", max_addr);
+
+       return 0;
+}
+early_param("max_addr", set_max_addr);
+
 void __init arm64_memblock_init(void)  {
        s64 linear_region_size = PAGE_END -
_PAGE_OFFSET(vabits_actual); @@ -253,6 +266,9 @@ void __init

arm64_memblock_init(void)

quoted

                memblock_add(__pa_symbol(_text), (u64)(_end -

_text));

quoted

        }

+       if (max_addr != PHYS_ADDR_MAX)
+               memblock_cap_memory_range(0, max_addr);
+
        if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size)

{

quoted

                /*
                 * Add back the memory we just removed if it results
in the @@ -427,4 +443,9 @@ void dump_mem_limit(void)
        } else {
                pr_emerg("Memory Limit: none\n");
        }
+
+       if (max_addr != PHYS_ADDR_MAX)
+               pr_emerg("Max addr: 0x%llx\n", max_addr);
+       else
+               pr_emerg("Max addr: none\n");
 }
--
2.25.1


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