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Re: [Qemu-devel] [PATCH] target-arm/helper.c: Don't flush the TLB if SCT
From: |
Laurent Desnogues |
Subject: |
Re: [Qemu-devel] [PATCH] target-arm/helper.c: Don't flush the TLB if SCTLR is rewritten unchanged |
Date: |
Thu, 8 May 2014 17:07:46 +0200 |
On Thu, May 8, 2014 at 4:40 PM, Peter Maydell <address@hidden> wrote:
> Linux makes a habit of writing the same value to the SCTLR that it
> already holds. In a sample boot of the kernel to a shell prompt
> it wrote the SCTLR with the value it already held 325465 times,
> and wrote different values just 3 times.
>
> Skip flushing the TLB if the SCTLR value isn't actually being changed;
> this speeds up my sample boot by 3-5%.
>
> Reported-by: Laurent Desnogues <address@hidden>
> Signed-off-by: Peter Maydell <address@hidden>
Reviewed-by: Laurent Desnogues <address@hidden>
Thanks,
Laurent
> ---
> I believe there are kernel patches in the works to avoid being
> quite so profligate with SCTLR writes, but there are still a
> lot of older kernels out in the world, so this is worth having IMHO.
>
> target-arm/helper.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/target-arm/helper.c b/target-arm/helper.c
> index 3be917c..417161e 100644
> --- a/target-arm/helper.c
> +++ b/target-arm/helper.c
> @@ -2081,6 +2081,13 @@ static void sctlr_write(CPUARMState *env, const
> ARMCPRegInfo *ri,
> {
> ARMCPU *cpu = arm_env_get_cpu(env);
>
> + if (env->cp15.c1_sys == value) {
> + /* Skip the TLB flush if nothing actually changed; Linux likes
> + * to do a lot of pointless SCTLR writes.
> + */
> + return;
> + }
> +
> env->cp15.c1_sys = value;
> /* ??? Lots of these bits are not implemented. */
> /* This may enable/disable the MMU, so do a TLB flush. */
> --
> 1.9.2
>