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

[Julien Grall <julien.grall@xxxxxxx>]

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 ...

> Currently, it is implemented for ARM64 only. Integrating the stuff
> with ARM32 requires moving ` if ( guest_mode(regs) )` condition
> into ARM32 specific traps.
... one more divergence between arm32 and arm64.

There are probably dozens of more conditions in the code that are not necessary 
for one of the architectures. Yet there are value to keep everything common 
because the benefits outweigh the likely non performance improvement.
So I am not convinced that this has any value for Xen.

Cheers,

--
Julien Grall

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