linux-kernel - Re: [RFC PATCH v2 04/23] KVM: TDX: Introduce tdx_clear

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Message-ID: <04d6d306-b495-428f-ac3a-44057fd6ccfc@linux.intel.com>
Date: Tue, 2 Sep 2025 10:56:25 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, x86@...nel.org, rick.p.edgecombe@...el.com,
 dave.hansen@...el.com, kas@...nel.org, tabba@...gle.com,
 ackerleytng@...gle.com, quic_eberman@...cinc.com, michael.roth@....com,
 david@...hat.com, vannapurve@...gle.com, vbabka@...e.cz,
 thomas.lendacky@....com, pgonda@...gle.com, zhiquan1.li@...el.com,
 fan.du@...el.com, jun.miao@...el.com, ira.weiny@...el.com,
 isaku.yamahata@...el.com, xiaoyao.li@...el.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH v2 04/23] KVM: TDX: Introduce tdx_clear_folio() to
 clear huge pages



On 8/7/2025 5:42 PM, Yan Zhao wrote:
> After removing or reclaiming a guest private page or a control page from a
> TD, zero the physical page using movdir64b(), enabling the kernel to reuse
> the pages.
>
> Introduce the function tdx_clear_folio() to zero out physical memory using
> movdir64b(), starting from the page at "start_idx" within a "folio" and
> spanning "npages" contiguous PFNs.
>
> Convert tdx_clear_page() to be a helper function to facilitate the
> zeroing of 4KB pages.

I think this sentence is outdated?

>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
> RFC v2:
> - Add tdx_clear_folio().
> - Drop inner loop _tdx_clear_page() and move __mb() outside of the loop.
>    (Rick)
> - Use C99-style definition of variables inside a for loop.
> - Note: [1] also changes tdx_clear_page(). RFC v2 is not based on [1] now.
>
> [1] https://lore.kernel.org/all/20250724130354.79392-2-adrian.hunter@intel.com
>
> RFC v1:
> - split out, let tdx_clear_page() accept level.
> ---
>   arch/x86/kvm/vmx/tdx.c | 22 ++++++++++++++++------
>   1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 8eaf8431c5f1..4fabefb27135 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -277,18 +277,21 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
>   	vcpu->cpu = -1;
>   }
>   
> -static void tdx_clear_page(struct page *page)
> +static void tdx_clear_folio(struct folio *folio, unsigned long start_idx,
> +			    unsigned long npages)
>   {
>   	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);
> +	for (unsigned long j = 0; j < npages; j++) {
> +		void *dest = page_to_virt(folio_page(folio, start_idx + j));
> +
> +		for (unsigned long 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.
> @@ -296,6 +299,13 @@ static void tdx_clear_page(struct page *page)
>   	__mb();
>   }
>   
> +static inline void tdx_clear_page(struct page *page)
No need to tag a local static function with "inline".

> +{
> +	struct folio *folio = page_folio(page);
> +
> +	tdx_clear_folio(folio, folio_page_idx(folio, page), 1);

This is strange at my first thought.
And then I realized that it is to avoid unnecessary memory barrier.

No better idea so far.
> +}
> +
>   static void tdx_no_vcpus_enter_start(struct kvm *kvm)
>   {
>   	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> @@ -1736,7 +1746,7 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
>   		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
>   		return -EIO;
>   	}
> -	tdx_clear_page(page);
> +	tdx_clear_folio(folio, folio_page_idx(folio, page), KVM_PAGES_PER_HPAGE(level));
>   	tdx_pamt_put(page, level);
>   	tdx_unpin(kvm, page);
>   	return 0;