Re: Proposal for Porting Xen to Armv8-R64 - DraftA

[Julien Grall <julien@xxxxxxx>]

Hi Wei,

On 03/03/2022 01:35, Wei Chen wrote:
> > > 1. Assembly code for EL1 MPU context_switch
> > This discussion reminds me when KVM decided to rewrite their context
> > switch from assembly to C. The outcome was the compiler is able to do a
> > better job than us when it comes to optimizing.
> >
> > With a C version, we could also share the save/restore code with 32-bit
> > and it is easier to read/maintain.
> >
> > So I would suggest to run some numbers to check if it really worth
> > implementing the MPU save/restore in assembly.
> It's interesting to hear KVM guys have similar discussion. Yes, if the
> gains of assembly code is not very obvious, then reusing the code for 32-bit
> would be more important. As our current platform (FVP) could not do very
> precise performance measurement. I want to keep current assembly code there,
> when we have a platform that can do such measurement we can have a thread
> to discuss it.
I briefly looked at the code, the assembly version is not going to be 
trivial to review and we don't know yet whether it has an advantage. So 
I would say this should be the inverse here.
We want the C version first until we can prove the assembly version is 
better.
My gut feeling is we will not need the assembly version.

Cheers,

--
Julien Grall