linux-kernel - Re: [PATCH v4 04/16] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

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Message-ID: <51dc2009-ff3e-4419-9dab-b46db7b2e15d@linux.intel.com>
Date: Tue, 25 Nov 2025 09:50:15 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: bp@...en8.de, chao.gao@...el.com, dave.hansen@...el.com,
 isaku.yamahata@...el.com, kai.huang@...el.com, kas@...nel.org,
 kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
 linux-kernel@...r.kernel.org, mingo@...hat.com, pbonzini@...hat.com,
 seanjc@...gle.com, tglx@...utronix.de, vannapurve@...gle.com,
 x86@...nel.org, yan.y.zhao@...el.com, xiaoyao.li@...el.com,
 binbin.wu@...el.com
Subject: Re: [PATCH v4 04/16] x86/virt/tdx: Allocate page bitmap for Dynamic
 PAMT



On 11/21/2025 8:51 AM, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>
> The Physical Address Metadata Table (PAMT) holds TDX metadata for physical
> memory and must be allocated by the kernel during TDX module
> initialization.
>
> The exact size of the required PAMT memory is determined by the TDX module
> and may vary between TDX module versions. Currently it is approximately
> 0.4% of the system memory. This is a significant commitment, especially if
> it is not known upfront whether the machine will run any TDX guests.
>
> For normal PAMT, each memory region that the TDX module might use (TDMR)
> needs three separate PAMT allocations. One for each supported page size
> (1GB, 2MB, 4KB).
>
> At a high level, Dynamic PAMT still has the 1GB and 2MB levels allocated
> on TDX module initialization, but the 4KB level allocated dynamically at
> TD runtime. However, in the details, the TDX module still needs some per
> 4KB page data. The TDX module exposed how many bits per page need to be
> allocated (currently it is 1). The bits-per-page value can then be used to
> calculate the size to pass in place of the 4KB allocations in the TDMR,
> which TDX specs call "PAMT_PAGE_BITMAP".
>
> So in effect, Dynamic PAMT just needs a different (smaller) size
> allocation for the 4KB level part of the allocation. Although it is
> functionally something different, it is passed in the same way the 4KB page
> size PAMT allocation is.
>
> Begin to implement Dynamic PAMT in the kernel by reading the bits-per-page
> needed for Dynamic PAMT. Calculate the size needed for the bitmap,
> and use it instead of the 4KB size determined for normal PAMT, in the case
> of Dynamic PAMT. In doing so, reduce the static allocations to
> approximately 0.004%, a 100x improvement.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> [Enhanced log]
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>

One nit below.

[...]
> diff --git a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
> index 13ad2663488b..00ab0e550636 100644
> --- a/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
> +++ b/arch/x86/virt/vmx/tdx/tdx_global_metadata.c
> @@ -33,6 +33,13 @@ static int get_tdx_sys_info_tdmr(struct tdx_sys_info_tdmr *sysinfo_tdmr)
>   		sysinfo_tdmr->pamt_2m_entry_size = val;
>   	if (!ret && !(ret = read_sys_metadata_field(0x9100000100000012, &val)))
>   		sysinfo_tdmr->pamt_1g_entry_size = val;
> +	/*
> +	 * Don't fail here if tdx_supports_dynamic_pamt() isn't supported. The

A bit weird to say "if tdx_supports_dynamic_pamt() isn't supported", how about
using "if dynamic PAMT isn't supported"?

> +	 * TDX code can fallback to normal PAMT if it's not supported.
> +	 */
> +	if (!ret && tdx_supports_dynamic_pamt(&tdx_sysinfo) &&
> +	    !(ret = read_sys_metadata_field(0x9100000100000013, &val)))
> +		sysinfo_tdmr->pamt_page_bitmap_entry_bits = val;
>   
>   	return ret;
>   }