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[FMV] Unify ssbs and ssbs2. #350

Merged
merged 3 commits into from
Oct 25, 2024
Merged

[FMV] Unify ssbs and ssbs2. #350

merged 3 commits into from
Oct 25, 2024

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labrinea
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According to https://developer.arm.com/documentation/102105/latest Arm Architecture Reference Manual for A-profile architecture: Known issues

2.206 D22789
In section C5.2.25 "SSBS, Speculative Store Bypass Safe", under the heading 'Configurations', the text that reads:

"This register is present only when FEAT_SSBS is implemented.
Otherwise, direct accesses to SSBS are UNDEFINED."

is changed to read:

"This register is present only when FEAT_SSBS2 is implemented.
Otherwise, direct accesses to SSBS are UNDEFINED."

This suggests that it's not worth splitting FEAT_SSBS2 from FEAT_SSBS in the compiler, since FEAT_SSBS cannot be used for predicating the MRS/MSR instructions. Those can access PSTATE.SSBS only when FEAT_SSBS2 is available. Moreover, there are no hardware implementations which implement FEAT_SSBS without FEAT_SSBS2, therefore unifying these features in the specification should not be a regression for feature detection.


name: Pull request
about: Technical issues, document format problems, bugs in scripts or feature proposal.


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According to https://developer.arm.com/documentation/102105/latest
Arm Architecture Reference Manual for A-profile architecture:
Known issues

2.206 D22789
In section C5.2.25 "SSBS, Speculative Store Bypass Safe",
under the heading 'Configurations', the text that reads:

"This register is present only when FEAT_SSBS is implemented.
 Otherwise, direct accesses to SSBS are UNDEFINED."

is changed to read:

"This register is present only when FEAT_SSBS2 is implemented.
 Otherwise, direct accesses to SSBS are UNDEFINED."

This suggests that it's not worth splitting FEAT_SSBS2 from FEAT_SSBS
in the compiler, since FEAT_SSBS cannot be used for predicating the
MRS/MSR instructions. Those can access PSTATE.SSBS only when FEAT_SSBS2
is available. Moreover, there are no hardware implementations which
implement FEAT_SSBS without FEAT_SSBS2, therefore unifying these
features in the specification should not be a regression for feature
detection.
@labrinea
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labrinea added a commit to labrinea/llvm-project that referenced this pull request Sep 27, 2024
According to https://developer.arm.com/documentation/102105/latest
Arm Architecture Reference Manual for A-profile architecture:
Known issues

2.206 D22789
In section C5.2.25 "SSBS, Speculative Store Bypass Safe",
under the heading 'Configurations', the text that reads:

"This register is present only when FEAT_SSBS is implemented.
Otherwise, direct accesses to SSBS are UNDEFINED."

is changed to read:

"This register is present only when FEAT_SSBS2 is implemented.
Otherwise, direct accesses to SSBS are UNDEFINED."

This suggests that it's not worth splitting FEAT_SSBS2 from FEAT_SSBS
in the compiler, since FEAT_SSBS cannot be used for predicating the
MRS/MSR instructions. Those can access PSTATE.SSBS only when FEAT_SSBS2
is available. Moreover, there are no hardware implementations which
implement FEAT_SSBS without FEAT_SSBS2, therefore unifying these
features in the specification should not be a regression for feature
detection.

Approved in ACLE as ARM-software/acle#350
labrinea added a commit to llvm/llvm-project that referenced this pull request Oct 7, 2024
According to https://developer.arm.com/documentation/102105/latest Arm
Architecture Reference Manual for A-profile architecture: Known issues

2.206 D22789
In section C5.2.25 "SSBS, Speculative Store Bypass Safe", under the
heading 'Configurations', the text that reads:

"This register is present only when FEAT_SSBS is implemented. Otherwise,
direct accesses to SSBS are UNDEFINED."

is changed to read:

"This register is present only when FEAT_SSBS2 is implemented.
Otherwise, direct accesses to SSBS are UNDEFINED."

This suggests that it's not worth splitting FEAT_SSBS2 from FEAT_SSBS in
the compiler, since FEAT_SSBS cannot be used for predicating the MRS/MSR
instructions. Those can access PSTATE.SSBS only when FEAT_SSBS2 is
available. Moreover, there are no hardware implementations which
implement FEAT_SSBS without FEAT_SSBS2, therefore unifying these
features in the specification should not be a regression for feature
detection.

Approved in ACLE as ARM-software/acle#350
@vhscampos vhscampos merged commit 4e28107 into ARM-software:main Oct 25, 2024
4 checks passed
@labrinea labrinea deleted the fmv-ssbs branch October 25, 2024 13:54
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3 participants