linux-kernel - Re: [PATCH 1/4] KVM: introduce CONFIG_KVM

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Date: Fri, 5 Jan 2024 21:28:09 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, ajones@...tanamicro.com
Subject: Re: [PATCH 1/4] KVM: introduce CONFIG_KVM_COMMON

On Fri, Jan 5, 2024 at 8:13 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > Start by introducing such a new Kconfig symbol, CONFIG_KVM_COMMON.
> > Unlike CONFIG_HAVE_KVM, it is selected by CONFIG_KVM, not by
> > architecture code.
>
> Why?  I don't get it, just have code that cares do IS_ENABLED(CONFIG_KVM).  Except
> for the MIPS usage of HAVE_KVM that you solved by adding CPU_SUPPORTS_VZ, I got
> all the way there using just CONFIG_KVM[*].
>
> Ah, and so does this series for the most part, the only usage of CONFIG_KVM_COMMON
> is in scripts/gdb/linux/constants.py.in.  Honestly, adding a Kconfig just so that
> VMX's posted interrupts that arrive in the host can be printed when KVM is built
> as a module is a waste of a Kconfig.

There is one extra thing that CONFIG_KVM_COMMON does, which is to
avoid having to select common requirements in all architectures.

I jotted this to solve the reported randconfig failure, which is why
CONFIG_KVM_COMMON only requires "select EVENTFD", but looking more
closely it should also select PREEMPT_NOTIFIERS and INTERVAL_TREE.
Both are used by virt/kvm/kvm_main.c, and loongarch + riscv both lack
INTERVAL_TREE so I do think it's a good idea to introduce this symbol
(though it requires a v2).

> [*] https://lore.kernel.org/all/20230916003118.2540661-12-seanjc@google.com

I guess you mean
https://lore.kernel.org/all/20230916003118.2540661-8-seanjc@google.com/.

Paolo