linux-kernel - RE: [PATCH] KVM: x86: optimize check for valid PAT value

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Date:   Wed, 10 Apr 2019 12:55:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Paolo Bonzini' <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH] KVM: x86: optimize check for valid PAT value

From: Paolo Bonzini
> Sent: 10 April 2019 10:55
> 
> This check will soon be done on every nested vmentry and vmexit,
> "parallelize" it using bitwise operations.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
...
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 28406aa1136d..7bc7ac9d2a44 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -347,4 +347,12 @@ static inline void kvm_after_interrupt(struct kvm_vcpu *vcpu)
>  	__this_cpu_write(current_vcpu, NULL);
>  }
> 
> +static inline bool kvm_pat_valid(u64 data)
> +{
> +	if (data & 0xF8F8F8F8F8F8F8F8)
> +		return false;
> +	/* 0, 1, 4, 5, 6, 7 are valid values.  */
> +	return (data | ((data & 0x0202020202020202) << 1)) == data;
> +}
> +

How about:
	/*
	 * Each byte must be 0, 1, 4, 5, 6 or 7.
	 * Convert 001x to 011x then 100x so 2 and 3 fail the test.
	 */
	data |= (data ^ 0x0404040404040404ULL)) + 0x0202020202020202ULL;
	if (data & 0xF8F8F8F8F8F8F8F8ULL)
		return false;

   David

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