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Differential D31014

pmap: On a failed fcmpset don't repeat tests that won't change
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Authored by alc on Jul 3 2021, 11:47 PM.
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rGe41fde3ed71c: On a failed fcmpset don't pointlessly repeat tests

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alc created this revision.Jul 3 2021, 11:47 PM
Herald added a reviewer: manu. · View Herald TranscriptJul 3 2021, 11:47 PM
Herald added subscribers: emaste, andrew. · View Herald Transcript
alc requested review of this revision.Jul 3 2021, 11:47 PM
alc added a comment.Jul 3 2021, 11:50 PM

On arm64, the new pmap code is smaller by 60 bytes.

kib accepted this revision.Jul 4 2021, 10:57 AM
This revision is now accepted and ready to land.Jul 4 2021, 10:57 AM
andrew added a comment.Jul 4 2021, 11:06 AM

Is it possible for the hardware to change the AF or DBM flags between loading the pte and the fcmpset in the arm64 pmap?

alc added a comment.Jul 4 2021, 10:32 PM
In D31014#697898, @andrew wrote:

Is it possible for the hardware to change the AF or DBM flags between loading the pte and the fcmpset in the arm64 pmap?

The hardware can change the AF bit from 0 to 1. The hardware does not change the DBM bit, only the OS does. If the DBM bit is 1, then the hardware can change ATTR_S1_AP(ATTR_S1_AP_RO) to ATTR_S1_AP(ATTR_S1_AP_RW) when the mapping is first written through, which amounts to changing a bit from 1 to 0.

markj accepted this revision.Jul 5 2021, 12:03 PM
andrew accepted this revision.Jul 5 2021, 7:32 PM
Closed by commit rGe41fde3ed71c: On a failed fcmpset don't pointlessly repeat tests (authored by alc). · Explain WhyJul 6 2021, 2:10 AM
This revision was automatically updated to reflect the committed changes.
alc added a commit: rGe41fde3ed71c: On a failed fcmpset don't pointlessly repeat tests.

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sys/
amd64/
amd64/
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arm64/
arm64/
15 lines

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sys/amd64/amd64/pmap.c

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sys/arm64/arm64/pmap.c

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