linux-kernel - Re: [PATCH 6/8] x86: add MULTIUSER dependency for KVM

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Message-ID: <8ee0dbeb-9a74-1e9a-2500-978f415c8b40@redhat.com>
Date:   Sun, 23 Jul 2017 15:41:58 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH 6/8] x86: add MULTIUSER dependency for KVM

On 19/07/2017 18:13, Radim Krčmář wrote:
> Good point, 'select' seems misused here.
> 
> There is no reason to depend on TASKSTATS (nor NET+MULTIUSER), we only
> suggest to enable it with KVM.  KVM uses sched_info_on() to handle any
> any possible resulting configuration, c9aaa8957f20 ("KVM: Steal time
> implementation").
> 
> KVM would work as intended if 'select' would not enable the option if
> its dependencies failed (instead of unconditionally forcing the option).
> 
> Is the preferred way to encode it:
> 
>   'default y if KVM' in config TASK_DELAY_ACCT
>   (that adds a non-local and enigmatic dependency and also needlessly
>    expands the possible configuration space)
> 
> or
> 
>   'select TASKSTATS if NET && MULTIUSER' in config KVM
>   (that is going to break when dependencies of TASKSTATS change again)
> 
> ?

I think the former is the closest to what the user actually wants, and
it would let us clean up arch/x86/kvm/Kconfig.  However it should be
"default y if KVM && X86'.

Maybe there is room for a new operator "suggest Y" which, when added
inside "config X", operates as if "config Y" had a "default y if X".

In this case, kvm could do

-	depends on NET && MULTIUSER
-	select TASKSTATS
+	suggest TASKSTATS

Thanks,

Paolo