linux-kernel - Re: [PATCH] acpi/hmat,mm/memtier: always register hmat adist calculation callback

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Message-ID: <878qxiowmy.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Wed, 31 Jul 2024 15:20:37 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Gregory Price <gourry@...rry.net>
Cc: linux-mm@...ck.org,  akpm@...ux-foundation.org,  dave.jiang@...el.com,
  Jonathan.Cameron@...wei.com,  horenchuang@...edance.com,
  linux-kernel@...r.kernel.org,  linux-acpi@...r.kernel.org,
  dan.j.williams@...el.com,  lenb@...nel.org,  "Aneesh Kumar K.V"
 <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH] acpi/hmat,mm/memtier: always register hmat adist
 calculation callback

Gregory Price <gourry@...rry.net> writes:

> On Wed, Jul 31, 2024 at 09:22:32AM +0800, Huang, Ying wrote:
>> Gregory Price <gourry@...rry.net> writes:
>> 
>> > This presumes driver configured devices, which is not always the case.
>> >
>> > kmem.c will set MEMTIER_DEFAULT_DAX_ADISTANCE
>> >
>> > but if BIOS/EFI has set up the node instead, you get the default of
>> > MEMTIER_ADISTANCE_DRAM if HMAT is not present or otherwise not sane.
>> 
>> "efi_fake_mem=" kernel parameter can be used to add "EFI_MEMORY_SP" flag
>> to the memory range, so that kmem.c can manage it.
>> 
>
> In this case, the system is configured explicitly so that kmem does not
> manage it. In fact, some systems still cannot be managed with
> EFI_MEMORY_SP due to hpa!=spa issues that the driver cannot manage.

Sorry, I don't understand.  IIUC, kmem.c can manage almost any memory
range via drivers/dax/hmem.  Please check

drivers/dax/hmem/device.c
drivers/dax/hmem/hmem.c

Could you elaborate why kmem.c doesn't work for some memory range?

>> > Not everyone is going to have the ability to get a platform vendor to
>> > fix a BIOS bug, and I've seen this in production.
>> 
>> So, some vendor build a machine with broken/missing HMAT/CDAT and wants
>> users to use CXL memory devices in it?  Have the vendor tested whether
>> CXL memory devices work?
>>
>
> As I mentioned, the broken aspect is being fixed, however there are
> existing production hardware which do not have HMAT entries.
>
>> > But the first step here would be creating two modes.  HMAT-is-sane and
>> > CPU/Non-CPU seems reasonable to me but open to opinions.
>> 
>> IMHO, we should reduce user configurable knobs unless we can prove it
>> is really necessary.
>>
>
> That's fair and valid.
>
> But I think a feature that worked in 5.x should work in 6.x, and right
> now the change in node placement breaks hardware that worked with 5.x
> which happened to have broken or missing HMAT.

--
Best Regards,
Huang, Ying