linux-kernel - Re: [PATCH 2/5] KVM: x86: Shrink struct kvm_queued

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Date:   Sat, 18 Feb 2023 08:55:38 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Mathias Krause <minipli@...ecurity.net>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/5] KVM: x86: Shrink struct kvm_queued_exception

On 2/13/23 10:33, Mathias Krause wrote:
> Reshuffle the boolean members of struct kvm_queued_exception and make
> them individual bits, allowing denser packing.
> 
> This allows us to shrink the object size from 24 to 16 bytes for 64 bit
> builds.
> 
> Signed-off-by: Mathias Krause <minipli@...ecurity.net>
> ---
>   arch/x86/include/asm/kvm_host.h | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 43329c60a6b5..040eee3e9583 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -701,13 +701,13 @@ struct kvm_vcpu_xen {
>   };
>   
>   struct kvm_queued_exception {
> -	bool pending;
> -	bool injected;
> -	bool has_error_code;
> +	u8 pending : 1;
> +	u8 injected : 1;
> +	u8 has_error_code : 1;
> +	u8 has_payload : 1;

I find doing something like this is clearer and easier to read:

	u8 pending		: 1,
	   injected		: 1,
	   has_error_code	: 1,
	   has_payload		: 1,
	   __reserved		: 4;

(you don't have to have the __reserved, though). Just throwing it out there.

Thanks,
Tom

>   	u8 vector;
>   	u32 error_code;
>   	unsigned long payload;
> -	bool has_payload;
>   };
>   
>   struct kvm_vcpu_arch {