Subject: Re: [PATCH RFC 0/7] Support KVM being compiled as a kernel module on arm64

From: Shannon Zhao <[email-masked]>
Date: Fri, 25 Oct 2019 10:24:14 +0800

Hi James,

On 2019/10/24 18:58, James Morse wrote:
> Hi Shannon,
> 
> On 24/10/2019 11:27, Shannon Zhao wrote:
>> Curently KVM ARM64 doesn't support to compile as a kernel module. It's
>> useful to compile KVM as a module.
> 
>> For example, it could reload kvm without rebooting host machine.
> 
> What problem does this solve?
> 
> KVM has some funny requirements that aren't normal for a module. On v8.0 hardware it must
> have an idmap. Modules don't usually expect their code to be physically contiguous, but
> KVM does. KVM influences they way some of the irqchip stuff is set up during early boot
> (EOI mode ... not that I understand it).
> 
> (I think KVM-as-a-module on x86 is an artifact of how it was developed)
> 
> 
>> This patchset support this feature while there are some limitations
>> to be solved. But I just send it out as RFC to get more suggestion and
>> comments.
> 
>> Curently it only supports for VHE system due to the hyp code section
>> address variables like __hyp_text_start.
> 
> We still need to support !VHE systems, and we need to do it with a single image.
> 
I didn't make it clear. With this patchset we still support !VHE systems 
by choose CONFIG_KVM_ARM_HOST as y and by default CONFIG_KVM_ARM_HOST is 
y. And during module init, I add a check to avoid wrong usage for kvm 
module.

if (IS_MODULE(CONFIG_KVM_ARM_HOST) && !is_kernel_in_hyp_mode()) {
         kvm_err("kvm arm kernel module only supports for VHE system\n");
         return -ENODEV;
}

> 
>> Also it can't call
>> kvm_update_va_mask when loading kvm module and kernel panic with below
>> errors. So I make kern_hyp_va into a nop funtion.
> 
> Making this work for the single-Image on v8.0 is going to be a tremendous amount of work.
> What is the payoff?
> 
Actually we can limit this feature only working for VHE systems and 
don't influence !VHE systems.

Thanks,
Shannon

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