Le 27/06/2025 à 00:01, Segher Boessenkool a écrit :
On Thu, Jun 26, 2025 at 07:56:10AM +0200, Christophe Leroy wrote:
Le 24/06/2025 à 23:08, David Laight a écrit :
On Tue, 24 Jun 2025 13:25:05 -0500
Segher Boessenkool <seg...@kernel.crashing.org> wrote:
isel (which is base PowerPC, not something "e500" only) is a
computational instruction, it copies one of two registers to a third,
which of the two is decided by any bit in the condition register.
Does that mean it could be used for all the ppc cpu variants?
No, only things that implement architecture version of 2.03 or later.
That is from 2006, so essentially everything that is still made
implements it :-)
But ancient things do not. Both 970 (Apple G5) and Cell BE do not yet
have it (they are ISA 2.01 and 2.02 respectively). And the older p5's
do not have it yet either, but the newer ones do.
For book3s64, GCC only use isel with -mcpu=power9 or -mcpu=power10
I have no idea what "book3s64" means.
Well that's the name given in Linux kernel to the 64 bits power CPU
processors. See commits subject:
f5164797284d book3s64/radix : Optimize vmemmap start alignment
58450938f771 book3s64/radix : Handle error conditions properly in
radix_vmemmap_populate
9cf7e13fecba book3s64/radix : Align section vmemmap start address to
PAGE_SIZE
29bdc1f1c1df book3s64/radix: Fix compile errors when
CONFIG_ARCH_WANT_OPTIMIZE_DAX_VMEMMAP=n
d629d7a8efc3 powerpc/book3s64/hugetlb: Fix disabling hugetlb when fadump
is active
5959ffabbb67 arch/powerpc: teach book3s64
arch_get_unmapped_area{_topdown} to handle hugetlb mappings
8846d9683884 book3s64/hash: Early detect debug_pagealloc size requirement
76b7d6463fc5 book3s64/hash: Disable kfence if not early init
b5fbf7e2c6a4 book3s64/radix: Refactoring common kfence related functions
8fec58f503b2 book3s64/hash: Add kfence functionality
47dd2e63d42a book3s64/hash: Disable debug_pagealloc if it requires more
memory
...
Some ancient Power architecture versions had something called
"Book III-S", which was juxtaposed to "Book III-E", which essentially
corresponds to the old aborted BookE stuff.
I guess you mean almost all non-FSL implementations? Most of those
support the isel insns. Like, Power5+ (GS). And everything after that.
I have no idea why you think power9 has it while older CPUS do not. In
the GCC source code we have this comment:
I think nothing, I just observed that GCC doesn't use it unless you tell
it is power9 or power10. But OK, the comment explains why.
/* For ISA 2.06, don't add ISEL, since in general it isn't a win, but
altivec is a win so enable it. */
and in fact we do not enable it for ISA 2.06 (p8) either, probably for
a similar reason.
And all classic PowerPC is ISA 1.xx of course. Medieval CPUs :-)
That make more sense than the list in patch 5/5.
Sorry for the ambiguity. In patch 5/5 I was addressing only powerpc/32,
and as far as I know the only powerpc/32 supported by Linux that has
isel is the 85xx which has an e500 core.
What is "powerpc/32"? It does not help if you use different names from
what everyone else does.
Again, that's the way it is called in Linux kernel, refer below commits
subjects:
$ git log --oneline arch/powerpc/ | grep powerpc/32
2bf3caa7cc3b powerpc/32: Stop printing Kernel virtual memory layout
2a17a5bebc9a powerpc/32: Replace mulhdu() by mul_u64_u64_shr()
dca5b1d69aea powerpc/32: Implement validation of emergency stack
2f2b9a3adc66 powerpc/32s: Reduce default size of module/execmem area
5799cd765fea powerpc/32: Convert patch_instruction() to patch_uint()
6035e7e35482 powerpc/32: Curb objtool unannotated intra-function call
warning
b72c066ba85a powerpc/32: fix ADB_CUDA kconfig warning
cb615bbe5526 powerpc/32: fix ADB_CUDA kconfig warning
c8a1634145c2 powerpc/32: Drop unused grackle_set_stg()
aad26d3b6af1 powerpc/32s: Implement local_flush_tlb_page_psize()
bac4cffc7c4a powerpc/32s: Introduce _PAGE_READ and remove _PAGE_USER
46ebef51fd92 powerpc/32s: Add _PAGE_WRITE to supplement _PAGE_RW
f84b727d132c powerpc/32: Enable POWER_RESET in pmac32_defconfig
a3ef2fef198c powerpc/32: Add dependencies of POWER_RESET for pmac32
7cb0094be4a5 powerpc/32s: Cleanup the mess in __set_pte_at()
6958ad05d578 powerpc/32: Rearrange _switch to prepare for 32/64 merge
fc8562c9b69a powerpc/32: Remove sync from _switch
...
It means everything built with CONFIG_PPC32
The name "powerpc32" is sometimes used colloquially to mean PowerPC code
running in SF=0 mode (MSR[SF]=0), but perhaps more often it is used for
32-bit only implementations (so, those that do not even have that bit:
it's bit 0 in the 64-bit MSR, so all implementations that have an only
32-bit MSR, for example).
For powerpc/64 we have less constraint than on powerpc32:
- Kernel memory starts at 0xc000000000000000
- User memory stops at 0x0010000000000000
That isn't true, not even if you mean some existing name. Usually
userspace code is mapped at 256MB (0x10000000). On powerpc64-linux
anyway, different default on different ABIs of course :-)
0x10000000 is below 0x0010000000000000, isn't it ? So why isn't it true ?
On 64 bits powerpc, everything related to user is between 0 and
TASK_SIZE_MAX.
TASK_SIZE_MAX is either TASK_SIZE_4PB or TASK_SIZE_64TB depending on
page size (64k or 4k)
TASK_SIZE_4PB is 0x0010000000000000UL
Christophe
And for access_ok() avoiding the conditional is a good enough reason
to use a 'conditional move' instruction.
Avoiding speculation is actually free.
And on CPUs that are not affected by Spectre and Meltdown like powerpc
8xx or powerpc 603,
Erm.
Segher
