Re: [PATCH] ACPI/HMAT: Fix the parsing of Cache Associativity and Write Policy

[Tao Xu]

On 12/11/2019 11:37 AM, Dan Williams wrote:
> On Tue, Dec 10, 2019 at 7:05 PM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > On 12/10/19 9:18 PM, Tao Xu wrote:
> > > On 12/10/2019 4:27 PM, Rafael J. Wysocki wrote:
> > > > On Tue, Dec 10, 2019 at 9:19 AM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > > > > On 12/10/2019 4:06 PM, Rafael J. Wysocki wrote:
> > > > > > On Tue, Dec 10, 2019 at 2:04 AM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > > > > > > On 12/9/2019 6:01 PM, Rafael J. Wysocki wrote:
> > > > > > > > On Mon, Dec 2, 2019 at 8:03 AM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > > > > > > > > In chapter 5.2.27.5, Table 5-147: Field "Cache Attributes" of
> > > > > > > > > ACPI 6.3 spec: 0 is "None", 1 is "Direct Mapped", 2 is "Complex
> > > > > > > > > Cache
> > > > > > > > > Indexing" for Cache Associativity; 0 is "None", 1 is "Write Back",
> > > > > > > > > 2 is "Write Through" for Write Policy.
> > > > > > > >
> > > > > > > > Well, I'm not sure what the connection between the above statement,
> > > > > > > > which is correct AFAICS, and the changes made by the patch is.
> > > > > > > >
> > > > > > > > Is that the *_OTHER symbol names are confusing or something deeper?
> > > > > > >
> > > > > > > Because in include/acpi/actbl1.h:
> > > > > > >
> > > > > > > #define ACPI_HMAT_CA_NONE                     (0)
> > > > > > >
> > > > > > > ACPI_HMAT_CA_NONE is 0, but in include/linux/node.h:
> > > > > > >
> > > > > > >        enum cache_indexing {
> > > > > > >               NODE_CACHE_DIRECT_MAP,
> > > > > > >               NODE_CACHE_INDEXED,
> > > > > > >               NODE_CACHE_OTHER,
> > > > > > >        };
> > > > > > > NODE_CACHE_OTHER is 2, and for otner enum:
> > > > > > >
> > > > > > >              case ACPI_HMAT_CA_DIRECT_MAPPED:
> > > > > > >                      tcache->cache_attrs.indexing =
> > > > > > > NODE_CACHE_DIRECT_MAP;
> > > > > > >                      break;
> > > > > > >              case ACPI_HMAT_CA_COMPLEX_CACHE_INDEXING:
> > > > > > >                      tcache->cache_attrs.indexing = NODE_CACHE_INDEXED;
> > > > > > >                      break;
> > > > > > > in include/acpi/actbl1.h:
> > > > > > >
> > > > > > >      #define ACPI_HMAT_CA_DIRECT_MAPPED            (1)
> > > > > > >      #define ACPI_HMAT_CA_COMPLEX_CACHE_INDEXING   (2)
> > > > > > >
> > > > > > > but in include/linux/node.h:
> > > > > > >
> > > > > > > NODE_CACHE_DIRECT_MAP is 0, NODE_CACHE_INDEXED is 1. This is
> > > > > > > incorrect.
> > > > > >
> > > > > > Why is it incorrect?
> > > > >
> > > > > Sorry I paste the wrong pre-define.
> > > > >
> > > > > This is the incorrect line:
> > > > >
> > > > > case ACPI_HMAT_CA_DIRECT_MAPPED:
> > > > > tcache->cache_attrs.indexing = NODE_CACHE_DIRECT_MAP;
> > > > >
> > > > > ACPI_HMAT_CA_DIRECT_MAPPED is 1, NODE_CACHE_DIRECT_MAP is 0. That means
> > > > > if HMAT table input 1 for cache_attrs.indexing, kernel store 0 in
> > > > > cache_attrs.indexing. But in ACPI 6.3, 0 means "None". So for the whole
> > > > > switch codes:
> > > >
> > > > This is a mapping between the ACPI-defined values and the generic ones
> > > > defined in the kernel.  There is not rule I know of by which they must
> > > > be the same numbers.  Or is there such a rule which I'm missing?
> > > >
> > > > As long as cache_attrs.indexing is used consistently going forward,
> > > > the difference between the ACPI-defined numbers and its values
> > > > shouldn't matter, should it?
> > > Yes, it will not influence the ACPI HMAT tables. Only influence is the
> > > sysfs, as in
> > > https://www.kernel.org/doc/html/latest/admin-guide/mm/numaperf.html:
> > >
> > > # tree sys/devices/system/node/node0/memory_side_cache/
> > > /sys/devices/system/node/node0/memory_side_cache/
> > > |-- index1
> > > |   |-- indexing
> > > |   |-- line_size
> > > |   |-- size
> > > |   `-- write_policy
> > >
> > > indexing is parsed in this file, so it can be read by user-space.
> > > Although now there is no user-space tool use this information to do some
> > > thing. But I am wondering if it is used in the future, someone use it to
> > > show the memory side cache information to user or use it to do
> > > performance turning.
> >
> > I finish a test using emulated ACPI HMAT from QEMU
> > (branch:hmat https://github.com/taoxu916/qemu.git)
> >
> > And I get the kernel log and sysfs output:
> > [    0.954288] HMAT: Cache: Domain:0 Size:20480 Attrs:00081111 SMBIOS
> > Handles:0
> > [    0.954835] HMAT: Cache: Domain:1 Size:15360 Attrs:00081111 SMBIOS
> > Handles:0
> >
> > /sys/devices/system/node/node0/memory_side_cache/index1 # cat indexing
> > 0
> > /sys/devices/system/node/node0/memory_side_cache/index1 # cat write_policy
> > 0
> >
> > Note that 'Attrs' is printed using %x, so we can get:
> > (attrs & ACPI_HMAT_CACHE_ASSOCIATIVITY) >> 8 = 1,
> > (attrs & ACPI_HMAT_WRITE_POLICY) >> 12       = 1
> >
> > but we get 0 in sysfs, so if user or software read this information and
> > read the ACPI 6.3 spec, will think there is 'none' for Cache
> > Associativity or Write Policy.
>
> The sysfs interface is not meant to reflect the ACPI values. This
> sysfs information may be populated by another platform firmware
> (non-ACPI). I would have preferred that these files use text values
> rather than numbers. However, at least the ABI documentation gives the
> expected translation:
>
> What:           /sys/devices/system/node/nodeX/memory_side_cache/indexY/indexing
> Date:           December 2018
> Contact:        Keith Busch <keith.busch@xxxxxxxxx>
> Description:
>                  The caches associativity indexing: 0 for direct mapped,
>                  non-zero if indexed.
>
> What:
> /sys/devices/system/node/nodeX/memory_side_cache/indexY/write_policy
> Date:           December 2018
> Contact:        Keith Busch <keith.busch@xxxxxxxxx>
> Description:
>                  The cache write policy: 0 for write-back, 1 for write-through,
>                  other or unknown.

I understand. Thank you for your explanation.