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

From: Julien Grall <julien@xen.org>
To: Wei Chen <Wei.Chen@arm.com>, Stefano Stabellini <sstabellini@kernel.org>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Bertrand Marquis <Bertrand.Marquis@arm.com>,
	Penny Zheng <Penny.Zheng@arm.com>,
	Henry Wang <Henry.Wang@arm.com>, nd <nd@arm.com>
Subject: Re: Proposal for Porting Xen to Armv8-R64 - DraftA
Date: Thu, 3 Mar 2022 09:15:27 +0000	[thread overview]
Message-ID: <4058b2c7-4c46-bdb1-017d-69a934989071@xen.org> (raw)
In-Reply-To: <PAXPR08MB7420836458E512FE375B8F199E049@PAXPR08MB7420.eurprd08.prod.outlook.com>

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