Hi Stefano,
On 24/05/18 00:47, Stefano Stabellini wrote:
On Tue, 22 May 2018, Julien Grall wrote:
Using current is fairly expensive, so save up into a variable.
Signed-off-by: Julien Grall <julien.gr...@arm.com>
Good idea. I am curious to know actually how much this patch would save
but I am not going to ask you run the tests.
I haven't benchmark it but looked at the resulting assembly code. This
reduces by about ~20% the number of instructions in the function.
AFAIU, this is because of the way per-cpu access have been implemented.
The per-cpu offset is stored in a system register (TPIDR_EL2), all the
read to it cannot be optimized (access using volatile).
So every direct use of "current" will require at least a system register
access and then a load from memory.
Cheers,
Reviewed-by: Stefano Stabellini <sstabell...@kernel.org>
---
xen/arch/arm/traps.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index 020b0b8eef..b1546f6907 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -2024,8 +2024,10 @@ static void enter_hypervisor_head(struct cpu_user_regs
*regs)
{
if ( guest_mode(regs) )
{
+ struct vcpu *v = current;
+
/* If the guest has disabled the workaround, bring it back on. */
- if ( needs_ssbd_flip(current) )
+ if ( needs_ssbd_flip(v) )
arm_smccc_1_1_smc(ARM_SMCCC_ARCH_WORKAROUND_2_FID, 1, NULL);
/*
@@ -2034,8 +2036,8 @@ static void enter_hypervisor_head(struct cpu_user_regs
*regs)
* but the crucial bit is "On taking a vSError interrupt, HCR_EL2.VSE
* (alias of HCR.VA) is cleared to 0."
*/
- if ( current->arch.hcr_el2 & HCR_VA )
- current->arch.hcr_el2 = READ_SYSREG(HCR_EL2);
+ if ( v->arch.hcr_el2 & HCR_VA )
+ v->arch.hcr_el2 = READ_SYSREG(HCR_EL2);
#ifdef CONFIG_NEW_VGIC
/*
@@ -2045,11 +2047,11 @@ static void enter_hypervisor_head(struct cpu_user_regs
*regs)
* TODO: Investigate whether this is necessary to do on every
* trap and how it can be optimised.
*/
- vtimer_update_irqs(current);
- vcpu_update_evtchn_irq(current);
+ vtimer_update_irqs(v);
+ vcpu_update_evtchn_irq(v);
#endif
- vgic_sync_from_lrs(current);
+ vgic_sync_from_lrs(v);
}
}
--
2.11.0
--
Julien Grall
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