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arm64: Exclude nohz_full CPUs from 32bits el0 support

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Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.

Suggested-by: Will Deacon <will@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
This commit is contained in:
Frederic Weisbecker 2024-10-25 00:12:59 +02:00
parent 192faebeb9
commit 4a1567b466
2 changed files with 17 additions and 1 deletions
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8
Documentation/arch/arm64/asymmetric-32bit.rst
View File

@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
mode will return to host userspace with an ``exit_reason`` of
``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
NOHZ FULL
---------
To avoid perturbing an adaptive-ticks CPU (specified using
``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
are treated as 64-bit-only when support for asymmetric 32-bit systems
is enabled.

10
arch/arm64/kernel/cpufeature.c
View File

@ -75,6 +75,7 @@
#include <linux/cpu.h>
#include <linux/kasan.h>
#include <linux/percpu.h>
#include <linux/sched/isolation.h>
#include <asm/cpu.h>
#include <asm/cpufeature.h>
@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
static int lucky_winner = -1;
struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
bool cpu_32bit = false;
if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
else
cpu_32bit = true;
}
if (cpu_32bit) {
cpumask_set_cpu(cpu, cpu_32bit_el0_mask);