Writes to ZCR_EL1 are self-synchronising, and so may be expensive
in typical implementations.
This patch adopts the approach used for costly system register
writes elsewhere in the kernel: the system register write is
suppressed if it would not change the stored value.
Since the common case will be that of switching between tasks that
use the same vector length as one another, prediction hit rates on
the conditional branch should be reasonably good, with lower
expected amortised cost than the unconditional execution of a
heavyweight self-synchronising instruction.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Manipulating the SVE architectural state, including the vector and
predicate registers, first-fault register and the vector length,
requires the use of dedicated instructions added by SVE.
This patch adds suitable assembly functions for saving and
restoring the SVE registers and querying the vector length.
Setting of the vector length is done as part of register restore.
Since people building kernels may not all get an SVE-enabled
toolchain for a while, this patch uses macros that generate
explicit opcodes in place of assembler mnemonics.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Support for kernel-mode NEON to be nested and/or used in hardirq
context adds significant complexity, and the benefits may be
marginal. In practice, kernel-mode NEON is not used in hardirq
context, and is rarely used in softirq context (by certain mac80211
drivers).
This patch implements an arm64 may_use_simd() function to allow
clients to check whether kernel-mode NEON is usable in the current
context, and simplifies kernel_neon_{begin,end}() to handle only
saving of the task FPSIMD state (if any). Without nesting, there
is no other state to save.
The partial fpsimd save/restore functions become redundant as a
result of these changes, so they are removed too.
The save/restore model is changed to operate directly on
task_struct without additional percpu storage. This simplifies the
code and saves a bit of memory, but means that softirqs must now be
disabled when manipulating the task fpsimd state from task context:
correspondingly, preempt_{en,dis}sable() calls are upgraded to
local_bh_{en,dis}able() as appropriate. fpsimd_thread_switch()
already runs with hardirqs disabled and so is already protected
from softirqs.
These changes should make it easier to support kernel-mode NEON in
the presence of the Scalable Vector extension in the future.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Alternate macro mode is not a property of a macro definition, but a
gas runtime state that alters the way macros are expanded for ever
after (until .noaltmacro is seen).
This means that subsequent assembly code that calls other macros can
break if fpsimdmacros.h is included.
Since these instruction sequences are simple (if dull -- but in a
good way), this patch solves the problem by simply expanding the
.irp loops. The pre-existing fpsimd_{save,restore} macros weren't
rolled with .irp anyway and the sequences affected are short, so
this change restores consistency at little cost.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Writing to the FPCR is commonly implemented as a self-synchronising
operation in the CPU, so avoid writing to the register when the saved
value matches that in the hardware already.
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This patch modifies kernel_neon_begin() and kernel_neon_end(), so
they may be called from any context. To address the case where only
a couple of registers are needed, kernel_neon_begin_partial(u32) is
introduced which takes as a parameter the number of bottom 'n' NEON
q-registers required. To mark the end of such a partial section, the
regular kernel_neon_end() should be used.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
In order to be able to reuse the save-restore code in KVM, move
it to a pair of macros, similar to what the 32bit code does.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
