RE: caching options

["Elliott, Robert (Persistent Memory)" <elliott@hpe.com>]

> -----Original Message-----
> From: Linux-nvdimm <linux-nvdimm-bounces@lists.01.org> On Behalf Of
> Brian Stark
> Sent: Tuesday, July 31, 2018 11:43 AM
> To: linux-nvdimm@lists.01.org
> Subject: caching options
> 
> I have a customer that requires persistent memory be set to Write
> Combining cache attribute with the Skylake processor.  Is Write
> Combining for persistent memory supported in Linux?
> 
> Brian Stark

There's no API for applications to specify that right now.

You can recompile the kernel with this patch:

$ git diff
diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
index 097072c5a852..d3f165d9ed77 100644
--- a/include/linux/libnvdimm.h
+++ b/include/linux/libnvdimm.h
@@ -206,7 +206,7 @@ int nvdimm_has_flush(struct nd_region *nd_region);
int nvdimm_has_cache(struct nd_region *nd_region);

#ifdef CONFIG_ARCH_HAS_PMEM_API
-#define ARCH_MEMREMAP_PMEM MEMREMAP_WB
+#define ARCH_MEMREMAP_PMEM MEMREMAP_WC
void arch_wb_cache_pmem(void *addr, size_t size);
void arch_invalidate_pmem(void *addr, size_t size);
#else

to cause persistent memory ranges used in "raw mode" to be marked as
write-combining rather than writeback cacheable. 

The other modes (sector, fsdax, and devdax) require different patches,
as the kernel maps memory differently for them.

You can confirm the write-combining settings if:
* the kernel is compiled with CONFIG_DEBUG_FS
* the debug filesystem is mounted with mount -t debugfs debugfs /sys/kernel/debug

$ sudo cat /sys/kernel/debug/x86/pat_memtype_list
PAT memtype list:
...
write-combining @ 0x880000000-0xa80000000
write-combining @ 0xa80000000-0xc80000000
...

The persistent memory addresses are reported in /proc/iomem
(e.g., starting at 0x880000000 on my test system).  

---
Robert Elliott, HPE Persistent Memory


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