linux-kernel - Re: [PATCH v2 5/9] KVM: x86/mmu: Factor out the meat of reset_tdp_shadow_zero_bits

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Message-ID: <YlWe6bwQX9V4Oc5S@google.com>
Date:   Tue, 12 Apr 2022 15:46:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v2 5/9] KVM: x86/mmu: Factor out the meat of
 reset_tdp_shadow_zero_bits_mask

On Mon, Mar 21, 2022, Ben Gardon wrote:
> Factor out the implementation of reset_tdp_shadow_zero_bits_mask to a
> helper function which does not require a vCPU pointer. The only element
> of the struct kvm_mmu context used by the function is the shadow root
> level, so pass that in too instead of the mmu context.
> 
> No functional change intended.
> 
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3b8da8b0745e..6f98111f8f8b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4487,16 +4487,14 @@ static inline bool boot_cpu_is_amd(void)
>   * possible, however, kvm currently does not do execution-protection.
>   */
>  static void

Strongly prefer the newline here get dropped (see below).

> -reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)
> +build_tdp_shadow_zero_bits_mask(struct rsvd_bits_validate *shadow_zero_check,

Kind of a nit, but KVM uses "calc" for this sort of thing.  There are no other
instances of "build_" to describe this behavior.

Am I alone in think that shadow_zero_check is an awful, awful name?  E.g. the EPT
memtype case has legal non-zero values.  Anyone object to opportunistically
renaming the function and the local shadow_zero_check to "rsvd_bits" to shorten
line lengths and move KVM one step closer to consistent naming?

> +				int shadow_root_level)
>  {
> -	struct rsvd_bits_validate *shadow_zero_check;
>  	int i;
>  
> -	shadow_zero_check = &context->shadow_zero_check;
> -
>  	if (boot_cpu_is_amd())
>  		__reset_rsvds_bits_mask(shadow_zero_check, reserved_hpa_bits(),
> -					context->shadow_root_level, false,
> +					shadow_root_level, false,
>  					boot_cpu_has(X86_FEATURE_GBPAGES),
>  					false, true);
>  	else
> @@ -4507,12 +4505,19 @@ reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)
>  	if (!shadow_me_mask)
>  		return;
>  
> -	for (i = context->shadow_root_level; --i >= 0;) {
> +	for (i = shadow_root_level; --i >= 0;) {
>  		shadow_zero_check->rsvd_bits_mask[0][i] &= ~shadow_me_mask;
>  		shadow_zero_check->rsvd_bits_mask[1][i] &= ~shadow_me_mask;
>  	}
>  }
>  
> +static void
> +reset_tdp_shadow_zero_bits_mask(struct kvm_mmu *context)

One line!  Aside from being against the One True Style[*], there is zero reason
for a newline here.

And I vote to drop the "mask", because (a) it's not a singular mask and (b) it's
not even a mask in all cases.

And while I'm on a naming consistency rant, s/context/mmu.

I.e. end up with:

static void calc_tdp_shadow_rsvd_bits(struct rsvd_bits_validate *rsvd_bits,
				      int shadow_root_level)

static void reset_tdp_shadow_rsvd_bits(struct kvm_mmu *mmu)

[*] https://lore.kernel.org/mm-commits/CAHk-=wjS-Jg7sGMwUPpDsjv392nDOOs0CtUtVkp=S6Q7JzFJRw@mail.gmail.com

> +{
> +	build_tdp_shadow_zero_bits_mask(&context->shadow_zero_check,
> +					context->shadow_root_level);
> +}
> +
>  /*
>   * as the comments in reset_shadow_zero_bits_mask() except it
>   * is the shadow page table for intel nested guest.
> -- 
> 2.35.1.894.gb6a874cedc-goog
>