linux-kernel - Re: [PATCH] KVM: x86/mmu: use helper macro SPTE_ENT_PER

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Date:   Wed, 5 Oct 2022 23:17:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: use helper macro SPTE_ENT_PER_PAGE

On Tue, Sep 13, 2022, Miaohe Lin wrote:
> Use helper macro SPTE_ENT_PER_PAGE to get the number of spte entries
> per page. Minor readability improvement.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 858bc53cfab4..45c532d00f78 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1645,7 +1645,7 @@ static int is_empty_shadow_page(u64 *spt)
>  	u64 *pos;
>  	u64 *end;
>  
> -	for (pos = spt, end = pos + PAGE_SIZE / sizeof(u64); pos != end; pos++)
> +	for (pos = spt, end = pos + SPTE_ENT_PER_PAGE; pos != end; pos++)

This is buried under MMU_DEBUG, and turning that on to compile test, which requires
manually changing kernel code to enable, results in some minor warnings.  Given the
existence of CONFIG_KVM_WERROR=y, I think it's safe to say this code hasn't been
exercised in a very long time.  E.g. this is literally the first time I've actually
enabled MMU_DEBUG.

This particular check seems like it would be quite useful, but the pgprintk() and
rmap_printk() hooks, not so much.  E.g. the knob is too coarse grained, and many
of the prints now have tracepoints.

So, unless someone actually actively uses MMU_DEBUG+dbg, I'm inclined to just delete
pgprintk() and rmap_printk(), and then rename MMU_WARN_ON => KVM_MMU_WARN_ON and
add a Kconfig for that, e.g. CONFIG_KVM_PROVE_MMU.

Hmm, and maybe clean up this helper too, e.g. get rid of the pointer arithmetic,
and take the full kvm_mmu_page so that the error message can print out things like
the gfn (a host kernel pointer is going to be useless for debug).

Thoughts?  Objections?