Re: [PATCH v2 05/21] x86/virt/seamldr: Introduce a wrapper for P-SEAMLDR SEAMCALLs

["Duan, Zhenzhong" <zhenzhong.duan@intel.com>]

On 10/1/2025 10:52 AM, Chao Gao wrote:
> Software needs to talk with P-SEAMLDR via P-SEAMLDR SEAMCALLs. So, add a
> wrapper for P-SEAMLDR SEAMCALLs.
>
> Save and restore the current VMCS using VMPTRST and VMPTRLD instructions
> to avoid breaking KVM. Doing so is because P-SEAMLDR SEAMCALLs would
> invalidate the current VMCS as documented in Intel® Trust Domain CPU
> Architectural Extensions (May 2021 edition) Chapter 2.3 [1]:
>
>    SEAMRET from the P-SEAMLDR clears the current VMCS structure pointed
>    to by the current-VMCS pointer. A VMM that invokes the P-SEAMLDR using
>    SEAMCALL must reload the current-VMCS, if required, using the VMPTRLD
>    instruction.
>
> Disable interrupts to prevent KVM code from interfering with P-SEAMLDR
> SEAMCALLs. For example, if a vCPU is scheduled before the current VMCS is
> restored, it may encounter an invalid current VMCS, causing its VMX
> instruction to fail. Additionally, if KVM sends IPIs to invalidate a
> current VMCS and the invalidation occurs right after the current VMCS is
> saved, that VMCS will be reloaded after P-SEAMLDR SEAMCALLs, leading to
> unexpected behavior.
>
> NMIs are not a problem, as the only scenario where instructions relying on
> the current-VMCS are used is during guest PMI handling in KVM. This occurs
> immediately after VM exits with IRQ and NMI disabled, ensuring no
> interference with P-SEAMLDR SEAMCALLs.
>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Tested-by: Farrah Chen <farrah.chen@intel.com>
> Link: https://cdrdv2.intel.com/v1/dl/getContent/733582 # [1]
> ---
> v2:
>   - don't create a new, inferior framework to save/restore VMCS
>   - use human-friendly language, just "current VMCS" rather than
>     SDM term "current-VMCS pointer"
>   - don't mix guard() with goto
> ---
>   arch/x86/Kconfig                | 10 ++++++
>   arch/x86/virt/vmx/tdx/Makefile  |  1 +
>   arch/x86/virt/vmx/tdx/seamldr.c | 56 +++++++++++++++++++++++++++++++++
>   3 files changed, 67 insertions(+)
>   create mode 100644 arch/x86/virt/vmx/tdx/seamldr.c
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 58d890fe2100..6b47383d2958 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1905,6 +1905,16 @@ config INTEL_TDX_HOST
>   
>   	  If unsure, say N.
>   
> +config INTEL_TDX_MODULE_UPDATE
> +	bool "Intel TDX module runtime update"
> +	depends on TDX_HOST_SERVICES
> +	help
> +	  This enables the kernel to support TDX module runtime update. This
> +	  allows the admin to update the TDX module to the same or any newer
> +	  version without the need to terminate running TDX guests.
> +
> +	  If unsure, say N.
> +
>   config EFI
>   	bool "EFI runtime service support"
>   	depends on ACPI
> diff --git a/arch/x86/virt/vmx/tdx/Makefile b/arch/x86/virt/vmx/tdx/Makefile
> index 90da47eb85ee..26aea3531c36 100644
> --- a/arch/x86/virt/vmx/tdx/Makefile
> +++ b/arch/x86/virt/vmx/tdx/Makefile
> @@ -1,2 +1,3 @@
>   # SPDX-License-Identifier: GPL-2.0-only
>   obj-y += seamcall.o tdx.o
> +obj-$(CONFIG_INTEL_TDX_MODULE_UPDATE) += seamldr.o

Not clear if seamldr will support other features besides TDX module update,

if yes, maybe more general name CONFIG_INTEL_SEAMLDR?

> diff --git a/arch/x86/virt/vmx/tdx/seamldr.c b/arch/x86/virt/vmx/tdx/seamldr.c
> new file mode 100644
> index 000000000000..b99d73f7bb08
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/seamldr.c
> @@ -0,0 +1,56 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright(c) 2025 Intel Corporation.
> + *
> + * Intel TDX module runtime update
> + */
> +#define pr_fmt(fmt)	"seamldr: " fmt
> +
> +#include <linux/irqflags.h>
> +#include <linux/types.h>
> +
> +#include "seamcall.h"
> +
> +static __maybe_unused int seamldr_call(u64 fn, struct tdx_module_args *args)
> +{
> +	unsigned long flags;
> +	u64 vmcs;
> +	int ret;
> +
> +	if (!is_seamldr_call(fn))
> +		return -EINVAL;
> +
> +	/*
> +	 * SEAMRET from P-SEAMLDR invalidates the current VMCS.  Save/restore
> +	 * the VMCS across P-SEAMLDR SEAMCALLs to avoid clobbering KVM state.
> +	 * Disable interrupts as KVM is allowed to do VMREAD/VMWRITE in IRQ
> +	 * context (but not NMI context).
> +	 */
> +	local_irq_save(flags);
> +
> +	asm goto("1: vmptrst %0\n\t"
> +		 _ASM_EXTABLE(1b, %l[error])
> +		 : "=m" (vmcs) : : "cc" : error);
> +
> +	ret = seamldr_prerr(fn, args);
> +
> +	/*
> +	 * Restore the current VMCS pointer.  VMPTSTR "returns" all ones if the
> +	 * current VMCS is invalid.
> +	 */
> +	if (vmcs != -1ULL) {
> +		asm goto("1: vmptrld %0\n\t"
> +			 "jna %l[error]\n\t"
> +			 _ASM_EXTABLE(1b, %l[error])
> +			 : : "m" (vmcs) : "cc" : error);
> +	}
> +
> +	local_irq_restore(flags);
> +	return ret;
> +
> +error:
> +	local_irq_restore(flags);
> +
> +	WARN_ONCE(1, "Failed to save/restore the current VMCS");
> +	return -EIO;
> +}