[v6,2/3] x86: probe memory block size advisement value during mm init

Commit Message

Gregory Price Nov. 6, 2024, 3:58 p.m. UTC

Systems with hotplug may provide an advisement value on what the
memblock size should be.  Probe this value when the rest of the
configuration values are considered.

The new heuristic is as follows

1) set_memory_block_size_order value if already set (cmdline param)
2) minimum block size if memory is less than large block limit
3) if no hotplug advice: Max block size if system is bare-metal,
   otherwise use end of memory alignment.
4) if hotplug advice: lesser of advice and end of memory alignment.

Convert to cpu_feature_enabled() while at it.[1]

[1] https://lore.kernel.org/all/20241031103401.GBZyNdGQ-ZyXKyzC_z@fat_crate.local/

Suggested-by: Borislav Petkov <[e-mail]>
Suggested-by: David Hildenbrand <[e-mail]>
Signed-off-by: Gregory Price <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Acked-by: Dave Hansen <[e-mail]>
Acked-by: Mike Rapoport (Microsoft) <[e-mail]>
---
 arch/x86/mm/init_64.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

Comments

Dan Williams Nov. 12, 2024, 9:34 p.m. UTC | #1

Gregory Price wrote:
> Systems with hotplug may provide an advisement value on what the
> memblock size should be.  Probe this value when the rest of the
> configuration values are considered.
> 
> The new heuristic is as follows
> 
> 1) set_memory_block_size_order value if already set (cmdline param)
> 2) minimum block size if memory is less than large block limit
> 3) if no hotplug advice: Max block size if system is bare-metal,
>    otherwise use end of memory alignment.
> 4) if hotplug advice: lesser of advice and end of memory alignment.

This seems like documentation that also belongs in-line in the code.
Perhaps a follow-on to add this to memory_block_advised_max_size()?
For this one:

Acked-by: Dan Williams <[e-mail]>

Patch

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index ff253648706f..2622dc7c78ba 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1452,16 +1452,21 @@  static unsigned long probe_memory_block_size(void)
 	}
 
 	/*
-	 * Use max block size to minimize overhead on bare metal, where
-	 * alignment for memory hotplug isn't a concern.
+	 * When hotplug alignment is not a concern, maximize blocksize
+	 * to minimize overhead. Otherwise, align to the lesser of advice
+	 * alignment and end of memory alignment.
 	 */
-	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
+	bz = memory_block_advised_max_size();
+	if (!bz) {
 		bz = MAX_BLOCK_SIZE;
-		goto done;
+		if (!cpu_feature_enabled(X86_FEATURE_HYPERVISOR))
+			goto done;
+	} else {
+		bz = max(min(bz, MAX_BLOCK_SIZE), MIN_MEMORY_BLOCK_SIZE);
 	}
 
 	/* Find the largest allowed block size that aligns to memory end */
-	for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
+	for (; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
 		if (IS_ALIGNED(boot_mem_end, bz))
 			break;
 	}