Re: [PATCH 00/24] alpha: catch up on architecture Kconfig options

[Matt Turner]

On Wed, Sep 2, 2026 at 2:54 PM Magnus Lindholm <linmag7@xxxxxxxxx> wrote:
>
> Hi Matt,
>
> On Tue, Sep 1, 2026 at 5:50 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
> >
> > Alpha is missing a long list of Kconfig options that other architectures
> > have picked up over the years, some of them for no better reason than
> > nobody having gone through the list.  I diffed the select statements in
> > every other arch/*/Kconfig against arch/alpha/Kconfig and worked through
> > what turned up.  This series is the result: two fixes it depends on, and
> > the options that are either free or close to it.
> >
> > The patches are ordered from least to most contentious, so the front of
> > the series can be applied on its own if the tail needs discussion.
> >
> > Patches 1-2 are fixes.  arch_irqs_disabled_flags() only reported
> > interrupts as disabled at IPL_MAX, but PALcode enters handlers at the
> > IPL of the interrupt being delivered, so a device or timer handler
> > answered that interrupts were enabled.  handle_irq() also called
> > irq_to_desc() before irq_enter(), running generic code with the preempt
> > count still saying task context and RCU not yet watching.  Both matter on
> > their own; patches 23 and 24 are what made them visible.
> >
> > Patches 3-11 are one-line selects with no Alpha code behind them:
> > instrumentation (UBSAN, gcov, kmemleak), EDAC, and four properties Alpha
> > has always had but never declared (LL/SC atomics, hardware multiply,
> > __int128, the existing asm/compiler.h, and the non-overlapping user and
> > kernel address spaces).
> >
> > Patches 12-17 add a small amount of code: an empty thread_struct
> > whitelist, _PAGE_SPECIAL on a free PTE bit, the page table check helpers,
> > and then the three things that follow from those - the page table helper
> > self test, lockless GUP, and the early memtest.  Note that early_memtest()
> > is called from arch_mm_preinit() rather than paging_init(), because Alpha
> > calls paging_init() from setup_arch(), before parse_early_param() has seen
> > the memtest= option.
> >
> > Patches 18-20 change something visible.  Unhandled user faults now print
> > the usual message under the debug.exception-trace sysctl.  .eh_frame is
> > discarded: gcc emits it for Alpha regardless of
> > -fno-asynchronous-unwind-tables, it was allocated in the loaded image, and
> > nothing consumes it, since the stack trace code scans for text addresses
> > rather than unwinding DWARF.  That is worth about 1.1MB.  With that and
> > the relocation sections placed explicitly, the kernel links with no
> > orphans left, so the last of the three asks the linker to say so.
> >
> > Patches 21-24 are the ones I expect to draw comment.  Patch 22 adds a
> > uapi header, so the perf register numbering is an ABI decision.  Patch 23
> > is SPARSE_IRQ: without it Marvel carries a 12.6MB static irq_desc[] in
> > .data for 32768 possible interrupts, of which a real ES47 uses 87, and
> > the kernel links at a fixed address just above the console reservation
> > where that 12.6MB is the difference between fitting and not.  Patch 24
> > instruments the entry assembly for lockdep hardirq tracking.
> >
> > Tested with gcc 17 under QEMU (-M clipper -smp 2, an ALPHA_GENERIC build)
> > with PROVE_LOCKING enabled: no splats, early_memtest passes all four
> > patterns over the free memblock ranges with nothing reserved,
> > debug_vm_pgtable validates with no warning, a vmsplice() of an anonymous
> > mapping through pin_user_pages_fast() returns the right data, and an
> > unhandled SIGSEGV prints the new trace line.  Patch 23 was tested on an
> > AlphaServer ES47.
> >
> > Not included, and worth mentioning: ARCH_HAS_KCOV.  A kernel with
> > KCOV_INSTRUMENT_ALL boots and runs, but CONFIG_KCOV_SELFTEST panics.  The
> > self test deliberately arms kcov with no coverage area so that any
> > instrumented function reached from interrupt context faults, checking
> > that nothing escapes the window before the preempt count says hardirq.
> > On Alpha something does.  Patch 2 fixes one such site; the panic survives
> > it, so the entry paths need an audit first.
> >
>
> Thanks for taking the time to do this! This is a nice collection of patches.
> I've started on the review and testing, I will keep at it.
> As general feedback for the series, I noticed that several of the
> corresponding architecture feature matrices still list Alpha as TODO.
> The Alpha entries should be changed to "ok" for:
>
>   ARCH_HAS_GCOV_PROFILE_ALL
>   HAVE_DEBUG_KMEMLEAK
>   ARCH_HAS_PTE_SPECIAL
>   ARCH_HAS_DEBUG_VM_PGTABLE
>   HAVE_PERF_REGS
>   HAVE_PERF_USER_STACK_DUMP
>
> These correspond to patches 4, 5, 13, 15, and 22.  It would probably be
> best to include each documentation update in the patch which enables the
> corresponding feature.

Oh, definitely. Thanks for catching that.

> It would also be helpful if the next revision were rebased onto
> v7.3-rc1 and applied cleanly on top of my Alpha GENERIC_ENTRY v5 series.
> I am currently testing only the patches which applied successfully.
>
> Alpha GENERIC_ENTRY v5 series is available here:
> https://lore.kernel.org/linux-alpha/20260902184058.198679-1-linmag7@xxxxxxxxx/T/#t

Perfect, I will do it!