Re: [RFC PATCH v2 02/23] x86/virt/tdx: Add SEAMCALL wrapper tdh_mem_page_demote()

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/7/2025 5:41 PM, Yan Zhao wrote:
> From: Xiaoyao Li <xiaoyao.li@intel.com>
>
> Introduce SEAMCALL wrapper tdh_mem_page_demote() to invoke the SEAMCALL
> TDH_MEM_PAGE_DEMOTE, which demotes a huge leaf entry to a non-leaf entry
> in the S-EPT.
>
> SEAMCALL TDH_MEM_PAGE_DEMOTE supports the demotion of 2MB or 1GB huge leaf
> entries.
>
> The "gpa" and "level" parameters enable the SEAMCALL TDH_MEM_PAGE_DEMOTE to
> walk the S-EPT for the huge leaf entry that needs to be demoted.
>
> The "page" parameter specifies a 4KB page that will be used in the demotion
> operation to be added as a page table page in the S-EPT.
>
> Invoke tdx_clflush_page() on the 4KB page being added as a page table page.
> This function performs CLFLUSH operations on certain TDX-capable platforms,
> or conservatively on all TDX-capable platforms, to prevent dirty cache
> lines from writing back later and corrupting TD memory.
>
> tdh_mem_page_demote() may fail. Callers can check function return value and
> retrieve extended error info from the function output parameters "ext_err1"
> and "ext_err2". e.g., due to S-EPT walk error or arriving interrupts.
>
> The TDX module has many internal locks. To avoid staying in SEAM mode for
> too long, SEAMCALLs return a BUSY error code to the kernel instead of
> spinning on the locks. Depending on the specific SEAMCALL, the caller may
> need to handle this error in specific ways (e.g., retry). Therefore, return
> the SEAMCALL error code directly to the caller without attempting to handle
> it in the core kernel.
>
> Do not handle TDX_INTERRUPTED_RESTARTABLE because SEAMCALL
> TDH_MEM_PAGE_DEMOTE does not check interrupts (including NMIs) for basic
> TDX (with or without Dynamic PAMT).

The cover letter mentions that there is a new TDX module in planning, which
disables the interrupt checking. I guess TDX module would need to have a
interface to report the change, KVM then decides to enable huge page support or
not for TDs?

>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Co-developed-by: Yan Zhao <yan.y.zhao@intel.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
> RFC v2:
> - Refine the patch log (Rick).
> - Do not handle TDX_INTERRUPTED_RESTARTABLE as the new TDX modules in
>    planning do not check interrupts for basic TDX.
>
> RFC v1:
> - Rebased and split patch. Updated patch log.
> ---
>   arch/x86/include/asm/tdx.h  |  2 ++
>   arch/x86/virt/vmx/tdx/tdx.c | 20 ++++++++++++++++++++
>   arch/x86/virt/vmx/tdx/tdx.h |  1 +
>   3 files changed, 23 insertions(+)
>
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index f968b736871a..d2cf48e273d5 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -178,6 +178,8 @@ u64 tdh_mng_key_config(struct tdx_td *td);
>   u64 tdh_mng_create(struct tdx_td *td, u16 hkid);
>   u64 tdh_vp_create(struct tdx_td *td, struct tdx_vp *vp);
>   u64 tdh_mng_rd(struct tdx_td *td, u64 field, u64 *data);
> +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *page,
> +			u64 *ext_err1, u64 *ext_err2);
>   u64 tdh_mr_extend(struct tdx_td *td, u64 gpa, u64 *ext_err1, u64 *ext_err2);
>   u64 tdh_mr_finalize(struct tdx_td *td);
>   u64 tdh_vp_flush(struct tdx_vp *vp);
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 580f14f64822..d941f083f741 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -1825,6 +1825,26 @@ u64 tdh_mng_rd(struct tdx_td *td, u64 field, u64 *data)
>   }
>   EXPORT_SYMBOL_GPL(tdh_mng_rd);
>   
> +u64 tdh_mem_page_demote(struct tdx_td *td, u64 gpa, int level, struct page *page,

Nit: Is it better to use a var name that clearly tell that the page is used as a
table page?

> +			u64 *ext_err1, u64 *ext_err2)
> +{
> +	struct tdx_module_args args = {
> +		.rcx = gpa | level,
> +		.rdx = tdx_tdr_pa(td),
> +		.r8 = page_to_phys(page),
> +	};
> +	u64 ret;
> +
> +	tdx_clflush_page(page);
> +	ret = seamcall_ret(TDH_MEM_PAGE_DEMOTE, &args);
> +
> +	*ext_err1 = args.rcx;
> +	*ext_err2 = args.rdx;
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(tdh_mem_page_demote);
> +
>   u64 tdh_mr_extend(struct tdx_td *td, u64 gpa, u64 *ext_err1, u64 *ext_err2)
>   {
>   	struct tdx_module_args args = {
> diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
> index 096c78a1d438..a6c0fa53ece9 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.h
> +++ b/arch/x86/virt/vmx/tdx/tdx.h
> @@ -24,6 +24,7 @@
>   #define TDH_MNG_KEY_CONFIG		8
>   #define TDH_MNG_CREATE			9
>   #define TDH_MNG_RD			11
> +#define TDH_MEM_PAGE_DEMOTE		15
>   #define TDH_MR_EXTEND			16
>   #define TDH_MR_FINALIZE			17
>   #define TDH_VP_FLUSH			18