Clean up ARM per-cpu qmap fields

[jah]

Summary

Remove qmap PTE and address fields for armv4, which I left unused
after r286296.

pc_qmap_addr for armv6 is a leftover from the old armv6 pmap, so might
as well rename it and put it to use in the new one.

[skra]

sys/arm/arm/pmap-v6.c

5894 ↗

(On Diff #24366)

Hmm. PCPU_GET() calls get_pcpu() internally, which is implemented as cp15 operation in SMP case. And cp15 operations may be slow. As an optimalization, I suggest to use get_pcpu() here and use obtained pointer.

Further, I checked other related functions. They use pcpu_find() when current struct pcpu* is wanted. While pcpu_find() is a function which needs to get current CPU id (one cp15 operation) to get struct pcpu*, get_pcpu() is a macro which gets struct pcpu* (one cp15 operation) directly. So, I suggest to replace pcpu_find() by get_pcpu() in these functions in a different commit.

[jah]

sys/arm/arm/pmap-v6.c

5894 ↗

(On Diff #24366)

Thank you for reminding me that we have get_pcpu() on arm. That was what I really wanted to use on i386, but unfortunately we only have the thread pointer in %gs there. BTW, concern over relative slowness of %gs and cp15 access was exactly the reason I chose to use pcpu_find() in the other functions: https://reviews.freebsd.org/D8833?id=23041#inline-52317

[jah]

sys/arm/arm/pmap-v6.c

5894 ↗

(On Diff #24366)

%gs->%fs

[skra]

sys/arm/arm/pmap-v6.c

5894 ↗

(On Diff #24366)

To clarify my note. The following line is used to get current pcpu in the other functions:
pc = pcpu_find(curcpu);
where curcpu is defined as
#define curcpu PCPU_GET(cpuid)

Thus, pcpu_find(curcpu) can be broken down as:
(1) get current pcpu
(2) get current cpu id from current pcpu
(3) call pcpu_find()
(4) get current pcpu according to current cpu id.

So, steps (2),(3), and (4) are pointless.

[jah]

sys/arm/arm/pmap-v6.c

5894 ↗

(On Diff #24366)

I understood your original note, I noted the same thing while working on the i386 pmap.
Looking at the code for each architecture, I don't see why we can't add get_pcpu() for the other architectures besides arm/arm64/riscv where it currently exists. That might be of some benefit since there are quite a few places in MI code where we use the pcpu_find(curcpu) trick.

BTW, I decided to include the switch to get_pcpu() with this commit since the changes are few and $(WORK) is leaving me with little time to do more freebsd work this week.

[jah]

Replace pcpu_find with get_pcpu()

[skra]

I suggest that the main reason for this change should be the replacement of pcpu_find() by get_pcpu() now. I mean the text in commit message. The cleanup should be stated as secondary.

It was my reason for separate commit. To clearly describe what was wrong with pcpu_find() .

sys/arm/arm/pmap-v6.c

5895 ↗

(On Diff #24437)

Type of pc_qmap_addr is vm_offset_t, and qmap_addr is used only once in this function. What about to not use qmap_addr variable here?

5894 ↗

(On Diff #24366)

I just wanted to be correct. The first note about pcpu_find() was not accurate.