linux-kernel - Re: [PATCH 02/14] KVM: monolithic: x86: disable linking vmx and svm at the same time into the kernel

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Date:   Mon, 14 Oct 2019 20:16:19 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 02/14] KVM: monolithic: x86: disable linking vmx and svm
 at the same time into the kernel

On Sat, Sep 28, 2019 at 01:23:11PM -0400, Andrea Arcangeli wrote:
> Linking both vmx and svm into the kernel at the same time isn't
> possible anymore or the kvm_x86/kvm_x86_pmu external function names
> would collide.
> 
> Reported-by: kbuild test robot <lkp@...el.com>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
> ---
>  arch/x86/kvm/Kconfig | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 840e12583b85..e1601c54355e 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -59,9 +59,29 @@ config KVM
>  
>  	  If unsure, say N.
>  
> +if KVM=y

Hmm, I see why the previous patch left KVM as a tristate.  I tried a
variety of hacks to let KVM be a bool but nothing worked.

> +
> +choice
> +	prompt "To link KVM statically into the kernel you need to choose"
> +	help
> +	  In order to build a kernel with support for both AMD and Intel
> +	  CPUs, you need to set CONFIG_KVM=m.
> +
> +config KVM_AMD_STATIC
> +	select KVM_AMD
> +	bool "Link KVM AMD statically into the kernel"
> +
> +config KVM_INTEL_STATIC
> +	select KVM_INTEL
> +	bool "Link KVM Intel statically into the kernel"

The prompt and choice text is way too long, e.g. in my usual window it
cuts off at:

  To link KVM statically into the kernel you need to choose (Link KVM Intel statically into

Without the full text (the -> at the end), it's not obvious it's an option
menu (AMD was selected by default for me and it took me a second to figure
out what to hit enter on).

I think short and sweet is enough for the prompt, with the details of how
build both buried in the help text.

choice
	prompt "KVM built-in support"
	help
	  Here be a long and detailed help text.

config KVM_AMD_STATIC
	select KVM_AMD
	bool "KVM AMD"

config KVM_INTEL_STATIC
	select KVM_INTEL
	bool "KVM Intel"

endchoice


The ends up looking like:

   <*>   Kernel-based Virtual Machine (KVM) support
           KVM built-in support (KVM Intel)  --->
   -*-   KVM for Intel processors support

> +
> +endchoice
> +
> +endif
> +
>  config KVM_INTEL
>  	tristate "KVM for Intel processors support"
> -	depends on KVM
> +	depends on (KVM && !KVM_AMD_STATIC) || KVM_INTEL_STATIC
>  	# for perf_guest_get_msrs():
>  	depends on CPU_SUP_INTEL
>  	---help---
> @@ -73,7 +93,7 @@ config KVM_INTEL
>  
>  config KVM_AMD
>  	tristate "KVM for AMD processors support"
> -	depends on KVM
> +	depends on (KVM && !KVM_INTEL_STATIC) || KVM_AMD_STATIC
>  	---help---
>  	  Provides support for KVM on AMD processors equipped with the AMD-V
>  	  (SVM) extensions.