| Message ID | 1399560029-19007-1-git-send-email-peter.maydell@linaro.org |
|---|---|
| State | New |
| Headers | show |
On Thu, May 8, 2014 at 4:40 PM, Peter Maydell <peter.maydell@linaro.org> 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 <laurent.desnogues@gmail.com> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com> 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 >
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. */
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 <laurent.desnogues@gmail.com> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> --- 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(+)