[PATCH v3 12/20] powerpc/pseries: add RTAS work area allocator
Nathan Lynch via B4 Submission Endpoint
devnull+nathanl.linux.ibm.com at kernel.org
Sat Feb 11 05:42:00 AEDT 2023
From: Nathan Lynch <nathanl at linux.ibm.com>
Various pseries-specific RTAS functions take a temporary "work area"
parameter - a buffer in memory accessible to RTAS. Typically such
functions are passed the statically allocated rtas_data_buf buffer as
the argument. This buffer is protected by a global spinlock. So users
of rtas_data_buf cannot perform sleeping operations while accessing
the buffer.
Most RTAS functions that have a work area parameter can return a
status (-2/990x) that indicates that the caller should retry. Before
retrying, the caller may need to reschedule or sleep (see
rtas_busy_delay() for details). This combination of factors
leads to uncomfortable constructions like this:
do {
spin_lock(&rtas_data_buf_lock);
rc = rtas_call(token, __pa(rtas_data_buf, ...);
if (rc == 0) {
/* parse or copy out rtas_data_buf contents */
}
spin_unlock(&rtas_data_buf_lock);
} while (rtas_busy_delay(rc));
Another unfortunately common way of handling this is for callers to
blithely ignore the possibility of a -2/990x status and hope for the
best.
If users were allowed to perform blocking operations while owning a
work area, the programming model would become less tedious and
error-prone. Users could schedule away, sleep, or perform other
blocking operations without having to release and re-acquire
resources.
We could continue to use a single work area buffer, and convert
rtas_data_buf_lock to a mutex. But that would impose an unnecessarily
coarse serialization on all users. As awkward as the current design
is, it prevents longer running operations that need to repeatedly use
rtas_data_buf from blocking the progress of others.
There are more considerations. One is that while 4KB is fine for all
current in-kernel uses, some RTAS calls can take much smaller buffers,
and some (VPD, platform dumps) would likely benefit from larger
ones. Another is that at least one RTAS function (ibm,get-vpd)
has *two* work area parameters. And finally, we should expect the
number of work area users in the kernel to increase over time as we
introduce lockdown-compatible ABIs to replace less safe use cases
based on sys_rtas/librtas.
So a special-purpose allocator for RTAS work area buffers seems worth
trying.
Properties:
* The backing memory for the allocator is reserved early in boot in
order to satisfy RTAS addressing requirements, and then managed with
genalloc.
* Allocations can block, but they never fail (mempool-like).
* Prioritizes first-come, first-serve fairness over throughput.
* Early boot allocations before the allocator has been initialized are
served via an internal static buffer.
Intended to replace rtas_data_buf. New code that needs RTAS work area
buffers should prefer this API.
Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
---
arch/powerpc/include/asm/rtas-work-area.h | 96 +++++++++++
arch/powerpc/kernel/rtas.c | 3 +
arch/powerpc/platforms/pseries/Makefile | 2 +-
arch/powerpc/platforms/pseries/rtas-work-area.c | 209 ++++++++++++++++++++++++
4 files changed, 309 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/rtas-work-area.h b/arch/powerpc/include/asm/rtas-work-area.h
new file mode 100644
index 000000000000..251a395dbd2e
--- /dev/null
+++ b/arch/powerpc/include/asm/rtas-work-area.h
@@ -0,0 +1,96 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_POWERPC_RTAS_WORK_AREA_H
+#define _ASM_POWERPC_RTAS_WORK_AREA_H
+
+#include <linux/build_bug.h>
+#include <linux/sizes.h>
+#include <linux/types.h>
+
+#include <asm/page.h>
+
+/**
+ * struct rtas_work_area - RTAS work area descriptor.
+ *
+ * Descriptor for a "work area" in PAPR terminology that satisfies
+ * RTAS addressing requirements.
+ */
+struct rtas_work_area {
+ /* private: Use the APIs provided below. */
+ char *buf;
+ size_t size;
+};
+
+enum {
+ /* Maximum allocation size, enforced at build time. */
+ RTAS_WORK_AREA_MAX_ALLOC_SZ = SZ_128K,
+};
+
+/**
+ * rtas_work_area_alloc() - Acquire a work area of the requested size.
+ * @size_: Allocation size. Must be compile-time constant and not more
+ * than %RTAS_WORK_AREA_MAX_ALLOC_SZ.
+ *
+ * Allocate a buffer suitable for passing to RTAS functions that have
+ * a memory address parameter, often (but not always) referred to as a
+ * "work area" in PAPR. Although callers are allowed to block while
+ * holding a work area, the amount of memory reserved for this purpose
+ * is limited, and allocations should be short-lived. A good guideline
+ * is to release any allocated work area before returning from a
+ * system call.
+ *
+ * This function does not fail. It blocks until the allocation
+ * succeeds. To prevent deadlocks, callers are discouraged from
+ * allocating more than one work area simultaneously in a single task
+ * context.
+ *
+ * Context: This function may sleep.
+ * Return: A &struct rtas_work_area descriptor for the allocated work area.
+ */
+#define rtas_work_area_alloc(size_) ({ \
+ static_assert(__builtin_constant_p(size_)); \
+ static_assert((size_) > 0); \
+ static_assert((size_) <= RTAS_WORK_AREA_MAX_ALLOC_SZ); \
+ __rtas_work_area_alloc(size_); \
+})
+
+/*
+ * Do not call __rtas_work_area_alloc() directly. Use
+ * rtas_work_area_alloc().
+ */
+struct rtas_work_area *__rtas_work_area_alloc(size_t size);
+
+/**
+ * rtas_work_area_free() - Release a work area.
+ * @area: Work area descriptor as returned from rtas_work_area_alloc().
+ *
+ * Return a work area buffer to the pool.
+ */
+void rtas_work_area_free(struct rtas_work_area *area);
+
+static inline char *rtas_work_area_raw_buf(const struct rtas_work_area *area)
+{
+ return area->buf;
+}
+
+static inline size_t rtas_work_area_size(const struct rtas_work_area *area)
+{
+ return area->size;
+}
+
+static inline phys_addr_t rtas_work_area_phys(const struct rtas_work_area *area)
+{
+ return __pa(area->buf);
+}
+
+/*
+ * Early setup for the work area allocator. Call from
+ * rtas_initialize() only.
+ */
+
+#ifdef CONFIG_PPC_PSERIES
+void rtas_work_area_reserve_arena(phys_addr_t limit);
+#else /* CONFIG_PPC_PSERIES */
+static inline void rtas_work_area_reserve_arena(phys_addr_t limit) {}
+#endif /* CONFIG_PPC_PSERIES */
+
+#endif /* _ASM_POWERPC_RTAS_WORK_AREA_H */
diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index a6d0c46b4512..1f80f0b8a9ad 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -36,6 +36,7 @@
#include <asm/machdep.h>
#include <asm/mmu.h>
#include <asm/page.h>
+#include <asm/rtas-work-area.h>
#include <asm/rtas.h>
#include <asm/time.h>
#include <asm/trace.h>
@@ -1939,6 +1940,8 @@ void __init rtas_initialize(void)
#endif
ibm_open_errinjct_token = rtas_token("ibm,open-errinjct");
ibm_errinjct_token = rtas_token("ibm,errinjct");
+
+ rtas_work_area_reserve_arena(rtas_region);
}
int __init early_init_dt_scan_rtas(unsigned long node,
diff --git a/arch/powerpc/platforms/pseries/Makefile b/arch/powerpc/platforms/pseries/Makefile
index 92310202bdd7..b63d7b4ba8fe 100644
--- a/arch/powerpc/platforms/pseries/Makefile
+++ b/arch/powerpc/platforms/pseries/Makefile
@@ -3,7 +3,7 @@ ccflags-$(CONFIG_PPC64) := $(NO_MINIMAL_TOC)
ccflags-$(CONFIG_PPC_PSERIES_DEBUG) += -DDEBUG
obj-y := lpar.o hvCall.o nvram.o reconfig.o \
- of_helpers.o \
+ of_helpers.o rtas-work-area.o \
setup.o iommu.o event_sources.o ras.o \
firmware.o power.o dlpar.o mobility.o rng.o \
pci.o pci_dlpar.o eeh_pseries.o msi.o \
diff --git a/arch/powerpc/platforms/pseries/rtas-work-area.c b/arch/powerpc/platforms/pseries/rtas-work-area.c
new file mode 100644
index 000000000000..1ee63335bd4b
--- /dev/null
+++ b/arch/powerpc/platforms/pseries/rtas-work-area.c
@@ -0,0 +1,209 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define pr_fmt(fmt) "rtas-work-area: " fmt
+
+#include <linux/genalloc.h>
+#include <linux/log2.h>
+#include <linux/kernel.h>
+#include <linux/memblock.h>
+#include <linux/mempool.h>
+#include <linux/minmax.h>
+#include <linux/mutex.h>
+#include <linux/numa.h>
+#include <linux/sizes.h>
+#include <linux/wait.h>
+
+#include <asm/machdep.h>
+#include <asm/rtas-work-area.h>
+#include <asm/rtas.h>
+
+enum {
+ /*
+ * Ensure the pool is page-aligned.
+ */
+ RTAS_WORK_AREA_ARENA_ALIGN = PAGE_SIZE,
+ /*
+ * Don't let a single allocation claim the whole arena.
+ */
+ RTAS_WORK_AREA_ARENA_SZ = RTAS_WORK_AREA_MAX_ALLOC_SZ * 2,
+ /*
+ * The smallest known work area size is for ibm,get-vpd's
+ * location code argument, which is limited to 79 characters
+ * plus 1 nul terminator.
+ *
+ * PAPR+ 7.3.20 ibm,get-vpd RTAS Call
+ * PAPR+ 12.3.2.4 Converged Location Code Rules - Length Restrictions
+ */
+ RTAS_WORK_AREA_MIN_ALLOC_SZ = roundup_pow_of_two(80),
+};
+
+static struct {
+ struct gen_pool *gen_pool;
+ char *arena;
+ struct mutex mutex; /* serializes allocations */
+ struct wait_queue_head wqh;
+ mempool_t descriptor_pool;
+ bool available;
+} rwa_state = {
+ .mutex = __MUTEX_INITIALIZER(rwa_state.mutex),
+ .wqh = __WAIT_QUEUE_HEAD_INITIALIZER(rwa_state.wqh),
+};
+
+/*
+ * A single work area buffer and descriptor to serve requests early in
+ * boot before the allocator is fully initialized. We know 4KB is the
+ * most any boot time user needs (they all call ibm,get-system-parameter).
+ */
+static bool early_work_area_in_use __initdata;
+static char early_work_area_buf[SZ_4K] __initdata __aligned(SZ_4K);
+static struct rtas_work_area early_work_area __initdata = {
+ .buf = early_work_area_buf,
+ .size = sizeof(early_work_area_buf),
+};
+
+
+static struct rtas_work_area * __init rtas_work_area_alloc_early(size_t size)
+{
+ WARN_ON(size > early_work_area.size);
+ WARN_ON(early_work_area_in_use);
+ early_work_area_in_use = true;
+ memset(early_work_area.buf, 0, early_work_area.size);
+ return &early_work_area;
+}
+
+static void __init rtas_work_area_free_early(struct rtas_work_area *work_area)
+{
+ WARN_ON(work_area != &early_work_area);
+ WARN_ON(!early_work_area_in_use);
+ early_work_area_in_use = false;
+}
+
+struct rtas_work_area * __ref __rtas_work_area_alloc(size_t size)
+{
+ struct rtas_work_area *area;
+ unsigned long addr;
+
+ might_sleep();
+
+ /*
+ * The rtas_work_area_alloc() wrapper enforces this at build
+ * time. Requests that exceed the arena size will block
+ * indefinitely.
+ */
+ WARN_ON(size > RTAS_WORK_AREA_MAX_ALLOC_SZ);
+
+ if (!rwa_state.available)
+ return rtas_work_area_alloc_early(size);
+ /*
+ * To ensure FCFS behavior and prevent a high rate of smaller
+ * requests from starving larger ones, use the mutex to queue
+ * allocations.
+ */
+ mutex_lock(&rwa_state.mutex);
+ wait_event(rwa_state.wqh,
+ (addr = gen_pool_alloc(rwa_state.gen_pool, size)) != 0);
+ mutex_unlock(&rwa_state.mutex);
+
+ area = mempool_alloc(&rwa_state.descriptor_pool, GFP_KERNEL);
+ area->buf = (char *)addr;
+ area->size = size;
+
+ return area;
+}
+
+void __ref rtas_work_area_free(struct rtas_work_area *area)
+{
+ if (!rwa_state.available) {
+ rtas_work_area_free_early(area);
+ return;
+ }
+
+ gen_pool_free(rwa_state.gen_pool, (unsigned long)area->buf, area->size);
+ mempool_free(area, &rwa_state.descriptor_pool);
+ wake_up(&rwa_state.wqh);
+}
+
+/*
+ * Initialization of the work area allocator happens in two parts. To
+ * reliably reserve an arena that satisfies RTAS addressing
+ * requirements, we must perform a memblock allocation early,
+ * immmediately after RTAS instantiation. Then we have to wait until
+ * the slab allocator is up before setting up the descriptor mempool
+ * and adding the arena to a gen_pool.
+ */
+static __init int rtas_work_area_allocator_init(void)
+{
+ const unsigned int order = ilog2(RTAS_WORK_AREA_MIN_ALLOC_SZ);
+ const phys_addr_t pa_start = __pa(rwa_state.arena);
+ const phys_addr_t pa_end = pa_start + RTAS_WORK_AREA_ARENA_SZ - 1;
+ struct gen_pool *pool;
+ const int nid = NUMA_NO_NODE;
+ int err;
+
+ err = -ENOMEM;
+ if (!rwa_state.arena)
+ goto err_out;
+
+ pool = gen_pool_create(order, nid);
+ if (!pool)
+ goto err_out;
+ /*
+ * All RTAS functions that consume work areas are OK with
+ * natural alignment, when they have alignment requirements at
+ * all.
+ */
+ gen_pool_set_algo(pool, gen_pool_first_fit_order_align, NULL);
+
+ err = gen_pool_add(pool, (unsigned long)rwa_state.arena,
+ RTAS_WORK_AREA_ARENA_SZ, nid);
+ if (err)
+ goto err_destroy;
+
+ err = mempool_init_kmalloc_pool(&rwa_state.descriptor_pool, 1,
+ sizeof(struct rtas_work_area));
+ if (err)
+ goto err_destroy;
+
+ rwa_state.gen_pool = pool;
+ rwa_state.available = true;
+
+ pr_debug("arena [%pa-%pa] (%uK), min/max alloc sizes %u/%u\n",
+ &pa_start, &pa_end,
+ RTAS_WORK_AREA_ARENA_SZ / SZ_1K,
+ RTAS_WORK_AREA_MIN_ALLOC_SZ,
+ RTAS_WORK_AREA_MAX_ALLOC_SZ);
+
+ return 0;
+
+err_destroy:
+ gen_pool_destroy(pool);
+err_out:
+ return err;
+}
+machine_arch_initcall(pseries, rtas_work_area_allocator_init);
+
+/**
+ * rtas_work_area_reserve_arena() - Reserve memory suitable for RTAS work areas.
+ */
+void __init rtas_work_area_reserve_arena(const phys_addr_t limit)
+{
+ const phys_addr_t align = RTAS_WORK_AREA_ARENA_ALIGN;
+ const phys_addr_t size = RTAS_WORK_AREA_ARENA_SZ;
+ const phys_addr_t min = MEMBLOCK_LOW_LIMIT;
+ const int nid = NUMA_NO_NODE;
+
+ /*
+ * Too early for a machine_is(pseries) check. But PAPR
+ * effectively mandates that ibm,get-system-parameter is
+ * present:
+ *
+ * R1–7.3.16–1. All platforms must support the System
+ * Parameters option.
+ *
+ * So set up the arena if we find that, with a fallback to
+ * ibm,configure-connector, just in case.
+ */
+ if (rtas_service_present("ibm,get-system-parameter") ||
+ rtas_service_present("ibm,configure-connector"))
+ rwa_state.arena = memblock_alloc_try_nid(size, align, min, limit, nid);
+}
--
2.39.1
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