linux-kernel - Re: [PATCH] KVM: x86/mmu: fix an incorrect comment in kvm_mmu_new

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Message-ID: <Y4T3xsxszDShKqK0@google.com>
Date:   Mon, 28 Nov 2022 18:02:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: fix an incorrect comment in kvm_mmu_new_pgd

On Sun, Nov 27, 2022, Wei Liu wrote:
> There is no function named kvm_mmu_ensure_valid_pgd.

() when referecing functions, here and in the comment.

> Fix the comment and remove the pair of braces to conform to Linux kernel
> coding style.
> 
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> ---
>  arch/x86/kvm/mmu/mmu.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b6f96d47e596..361574124fbe 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4452,10 +4452,12 @@ void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd)
>  	struct kvm_mmu *mmu = vcpu->arch.mmu;
>  	union kvm_mmu_page_role new_role = mmu->root_role;
>  
> -	if (!fast_pgd_switch(vcpu->kvm, mmu, new_pgd, new_role)) {
> -		/* kvm_mmu_ensure_valid_pgd will set up a new root.  */
> +	/*
> +	 * Return immediately if no usable root is found. A new root will be

s/is/was

And technically, it may not be a "new" root, e.g. if a different vCPU creates a
usable root between now and kvm_mmu_reload().

> +	 * set up in vcpu_enter_guest prior to the next vmenter.

s/vmenter/VM-Enter

Pointing a reader at vcpu_enter_guest() isn't very helpful since that's a massive
function; kvm_mmu_reload() is where the real magic happens.  I generally prefer to
avoid referencing functions in comments unless doing so is absolutely "necessary"
because such comments always seem to become stale, but in this case I think it's
worthwhile.

	/*
	 * Return immediately if no usable was found, kvm_mmu_reload() will
	 * establish a valid root prior to the next VM-Enter.
	 */

> +	 */
> +	if (!fast_pgd_switch(vcpu->kvm, mmu, new_pgd, new_role))
>  		return;
> -	}
>  
>  	/*
>  	 * It's possible that the cached previous root page is obsolete because
> -- 
> 2.35.1
>