Re: [Xen-devel] [RFC 5/6] arm64: call enter_hypervisor_head only when it is needed

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrii,

On 30/07/2019 18:35, Andrii Anisov wrote:
> On 26.07.19 13:59, Julien Grall wrote:
> > Hi,
> >
> > On 26/07/2019 11:37, Andrii Anisov wrote:
> > > From: Andrii Anisov <andrii_anisov@xxxxxxxx>
> > >
> > > On ARM64 we know exactly if trap happened from hypervisor or guest, so
> > > we do not need to take that decision. This reduces a condition for
> > > all enter_hypervisor_head calls and the function call for traps from
> > > the hypervisor mode.
> > One condition lost but ...
> ...In the hot path (actually at any trap).
Everything is in the hot path here, yet there are a lot of other branches. So 
why this branch in particular?
As I have mentioned a few times before, there are a difference between the 
theory and the practice. In theory, removing a branch looks nice. But in 
practice this may not be the case.
In this particular case, I don't believe this is going to have a real impact on 
the performance.
The PSTATE has been saved a few instructions before in cpu_user_regs, so there
are high chance the value will still be in the L1 cache.

The compiler may also decide to do the direct branch when not in guest_mode. A 
trap from the hypervisor is mostly for interrupts. So there are chance this is 
not going to have a real impact on the overall of the interrupt handling.
If you are really worry of the impact of branch then there are a few more 
important places (with a greater benefits) to look:
    1) It seems the compiler use a jump table for the switch case in 
do_trap_guest_sync(), so it will result to multiple direct branch everytime.
    2) Indirect branch have a non-negligible cost compare to direct branch. 
This is a lot used in the interrupt code (see gic_hw_ops->read_irq()). All of 
them are known at boot time, so they could be replace with direct branch. x86 
recently introduced alternative_call() for this purpose. This could be re-used 
on Arm.
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel