linux-kernel - Re: [PATCH v3 05/16] x86/virt/tdx: Allocate reference counters for PAMT memory

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Message-ID: <47f8b553-1cb0-4fb0-993b-affd4468475e@linux.intel.com>
Date: Tue, 23 Sep 2025 15:45:49 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: kas@...nel.org, bp@...en8.de, chao.gao@...el.com,
 dave.hansen@...ux.intel.com, isaku.yamahata@...el.com, kai.huang@...el.com,
 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, x86@...nel.org, yan.y.zhao@...el.com,
 vannapurve@...gle.com, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 05/16] x86/virt/tdx: Allocate reference counters for
 PAMT memory



On 9/19/2025 7:22 AM, Rick Edgecombe wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>
> The PAMT memory holds metadata for TDX protected memory. With Dynamic
> PAMT, the 4KB range of PAMT is allocated on demand. The kernel supplies
> the TDX module with a page pair that covers 2MB of host physical memory.
>
> The kernel must provide this page pair before using pages from the range
> for TDX. If this is not done, any SEAMCALL that attempts to use the memory
> will fail.
>
> Allocate reference counters for every 2MB range to track PAMT memory usage.
> This is necessary to accurately determine when PAMT memory needs to be
> allocated and when it can be freed.
>
> This allocation will currently consume 2 MB for every 1 TB of address
> space from 0 to max_pfn (highest pfn of RAM). The allocation size will
> depend on how the ram is physically laid out. In a worse case scenario
> where the entire 52 address space is covered this would be 8GB. Then
                   ^
                  52-bit
> the DPAMT refcount allocations could hypothetically exceed the savings
> from Dynamic PAMT, which is 4GB per TB. This is probably unlikely.
>
> However, future changes will reduce this refcount overhead to make DPAMT
> always a net win.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> [Add feedback, update log]
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
> v3:
>   - Split out lazily populate optimization to next patch (Dave)
>   - Add comment around pamt_refcounts (Dave)
>   - Improve log
> ---
>   arch/x86/virt/vmx/tdx/tdx.c | 47 ++++++++++++++++++++++++++++++++++++-
>   1 file changed, 46 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 4e4aa8927550..0ce4181ca352 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -29,6 +29,7 @@
>   #include <linux/acpi.h>
>   #include <linux/suspend.h>
>   #include <linux/idr.h>
> +#include <linux/vmalloc.h>
>   #include <asm/page.h>
>   #include <asm/special_insns.h>
>   #include <asm/msr-index.h>
> @@ -50,6 +51,16 @@ static DEFINE_PER_CPU(bool, tdx_lp_initialized);
>   
>   static struct tdmr_info_list tdx_tdmr_list;
>   
> +/*
> + * On a machine with Dynamic PAMT, the kernel maintains a reference counter
> + * for every 2M range. The counter indicates how many users there are for
> + * the PAMT memory of the 2M range.
> + *
> + * The kernel allocates PAMT memory when the first user arrives and
> + * frees it when the last user has left.
> + */
> +static atomic_t *pamt_refcounts;
> +
>   static enum tdx_module_status_t tdx_module_status;
>   static DEFINE_MUTEX(tdx_module_lock);
>   
> @@ -183,6 +194,34 @@ int tdx_cpu_enable(void)
>   }
>   EXPORT_SYMBOL_GPL(tdx_cpu_enable);
>   
> +/*
> + * Allocate PAMT reference counters for all physical memory.
> + *
> + * It consumes 2MiB for every 1TiB of physical memory.
> + */
> +static int init_pamt_metadata(void)
> +{
> +	size_t size = max_pfn / PTRS_PER_PTE * sizeof(*pamt_refcounts);

Is there guarantee that max_pfn is PTRS_PER_PTE aligned?
If not, it should be rounded up.

> +
> +	if (!tdx_supports_dynamic_pamt(&tdx_sysinfo))
> +		return 0;
> +
> +	pamt_refcounts = vmalloc(size);
> +	if (!pamt_refcounts)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
>
[...]