linux-kernel - Re: [PATCH V3 1/2] x86/tdx: Eliminate duplicate code in tdx_clear

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Message-ID: <aH-b5UAkokFocLvG@google.com>
Date: Tue, 22 Jul 2025 07:11:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, pbonzini@...hat.com, vannapurve@...gle.com, 
	Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org, H Peter Anvin <hpa@...or.com>, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, rick.p.edgecombe@...el.com, 
	kas@...nel.org, kai.huang@...el.com, reinette.chatre@...el.com, 
	xiaoyao.li@...el.com, tony.lindgren@...ux.intel.com, 
	binbin.wu@...ux.intel.com, isaku.yamahata@...el.com, yan.y.zhao@...el.com, 
	chao.gao@...el.com
Subject: Re: [PATCH V3 1/2] x86/tdx: Eliminate duplicate code in tdx_clear_page()

On Tue, Jul 22, 2025, Adrian Hunter wrote:
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 7ddef3a69866..f66328404724 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -131,6 +131,8 @@ int tdx_guest_keyid_alloc(void);
>  u32 tdx_get_nr_guest_keyids(void);
>  void tdx_guest_keyid_free(unsigned int keyid);
>  
> +void tdx_quirk_reset_paddr(unsigned long base, unsigned long size);
> +
>  struct tdx_td {
>  	/* TD root structure: */
>  	struct page *tdr_page;
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 573d6f7d1694..1b549de6da06 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -283,25 +283,6 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
>  	vcpu->cpu = -1;
>  }
>  
> -static void tdx_clear_page(struct page *page)
> -{
> -	const void *zero_page = (const void *) page_to_virt(ZERO_PAGE(0));
> -	void *dest = page_to_virt(page);
> -	unsigned long i;
> -
> -	/*
> -	 * The page could have been poisoned.  MOVDIR64B also clears
> -	 * the poison bit so the kernel can safely use the page again.
> -	 */
> -	for (i = 0; i < PAGE_SIZE; i += 64)
> -		movdir64b(dest + i, zero_page);
> -	/*
> -	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
> -	 * from seeing potentially poisoned cache.
> -	 */
> -	__mb();
> -}
> -
>  static void tdx_no_vcpus_enter_start(struct kvm *kvm)
>  {
>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> @@ -347,7 +328,7 @@ static int tdx_reclaim_page(struct page *page)
>  
>  	r = __tdx_reclaim_page(page);
>  	if (!r)
> -		tdx_clear_page(page);
> +		tdx_quirk_reset_paddr(page_to_phys(page), PAGE_SIZE);

This is silly.  Literally every use in KVM is on a struct page.  I agree with
Dave that having a wrapper with a completely unrelated name is confusing, but
that's a naming problem, not a code problem.

And FWIW, I find tdx_quirk_reset_paddr() confusing, because it reads like it's
resetting the address itself.  But if KVM only ever uses tdx_quirk_reset_page(),
I don't care what you call the inner helper.