linux-kernel - Re: [RFC PATCH v1] KVM: x86: Introduce macros to simplify KVM_X86

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Message-ID: <Zh_4QN7eFxyu9hgA@google.com>
Date: Wed, 17 Apr 2024 09:26:40 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Wei Wang <wei.w.wang@...el.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1] KVM: x86: Introduce macros to simplify KVM_X86_OPS
 static calls

On Wed, Apr 17, 2024, Wei Wang wrote:
> Introduces two new macros, KVM_X86_SC() and KVM_X86_SCC(), to streamline
> the usage of KVM_X86_OPS static calls. The current implementation of these
> calls is verbose and can lead to alignment challenges due to the two pairs
> of parentheses. This makes the code susceptible to exceeding the "80
> columns per single line of code" limit as defined in the coding-style
> document. The two macros are added to improve code readability and
> maintainability, while adhering to the coding style guidelines.

Heh, I've considered something similar on multiple occasionsi.  Not because
the verbosity bothers me, but because I often search for exact "word" matches
when looking for function usage and the kvm_x86_ prefix trips me up.

> Please note that this RFC only updated a few callsites for demonstration
> purposes. If the approach looks good, all callsites will be updated in
> the next version.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  3 +++
>  arch/x86/kvm/lapic.c            | 15 ++++++++-------
>  arch/x86/kvm/trace.h            |  3 ++-
>  arch/x86/kvm/x86.c              |  8 ++++----
>  4 files changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 6efd1497b026..42f6450c10ec 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1856,6 +1856,9 @@ extern struct kvm_x86_ops kvm_x86_ops;
>  	DECLARE_STATIC_CALL(kvm_x86_##func, *(((struct kvm_x86_ops *)0)->func));
>  #define KVM_X86_OP_OPTIONAL KVM_X86_OP
>  #define KVM_X86_OP_OPTIONAL_RET0 KVM_X86_OP
> +
> +#define KVM_X86_SC(func, ...) static_call(kvm_x86_##func)(__VA_ARGS__)
> +#define KVM_X86_SCC(func, ...) static_call_cond(kvm_x86_##func)(__VA_ARGS__)

IIRC, static_call_cond() is essentially dead code, i.e. it's the exact same as
static_call().  I believe there's details buried in a proposed series to remove
it[*].  And to not lead things astray, I verified that invoking a NULL kvm_x86_op
with static_call() does no harm (well, doesn't explode at least).

So if we add wrapper macros, I would be in favor in removing all static_call_cond()
as a prep patch so that we can have a single macro.  kvm_ops_update() already WARNs
if a mandatory hook isn't defined, so doing more checks at runtime wouldn't provide
any value.

As for the name, what about KVM_X86_CALL() instead of KVM_X86_SC()?  Two extra
characters, but should make it much more obvious what's going on for readers that
aren't familiar with the infrastructure.

And I bet we can get away with KVM_PMU_CALL() for the PMU hooks.

[*] https://lore.kernel.org/all/cover.1679456900.git.jpoimboe@kernel.org