Re: [RFC PATCH 03/20] x86/virt/seamldr: Introduce a wrapper for P-SEAMLDR SEAMCALLs

[Nikolay Borisov]

On 5/23/25 12:52, Chao Gao wrote:
> P-SEAMLDR is another component alongside the TDX module within the
> protected SEAM range. Software can invoke its functions by executing the
> SEAMCALL instruction with the 63 bit of RAX set to 1. P-SEAMLDR SEAMCALLs
> differ from those of the TDX module in terms of error codes and the
> handling of the current VMCS.
>
> Add a wrapper for P-SEAMLDR SEAMCALLs based on the SEAMCALL infrastructure.
>
> Intel® Trust Domain CPU Architectural Extensions (May 2021 edition)
> Chapter 2.3 states:
>
> 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.
>
> So, save and restore the current-VMCS pointer using VMPTRST and VMPTRLD
> instructions to avoid breaking KVM, which manages the current-VMCS.
>
> 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@xxxxxxxxx>
> Tested-by: Farrah Chen <farrah.chen@xxxxxxxxx>
> ---
>   arch/x86/Kconfig                | 10 ++++++++
>   arch/x86/virt/vmx/tdx/Makefile  |  1 +
>   arch/x86/virt/vmx/tdx/seamldr.c | 44 +++++++++++++++++++++++++++++++++
>   arch/x86/virt/vmx/vmx.h         | 40 ++++++++++++++++++++++++++++++
>   4 files changed, 95 insertions(+)
>   create mode 100644 arch/x86/virt/vmx/tdx/seamldr.c
>   create mode 100644 arch/x86/virt/vmx/vmx.h
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4b9f378e05f6..8b1e0986b7f8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1932,6 +1932,16 @@ config INTEL_TDX_HOST
>   
>   	  If unsure, say N.
>   
> +config INTEL_TDX_MODULE_UPDATE
> +	bool "Intel TDX module runtime update"
> +	depends on INTEL_TDX_HOST
> +	help
> +	  This enables the kernel to support TDX module runtime update. This allows
> +	  the admin to upgrade the TDX module to a newer one without the need to
> +	  terminate running TDX guests.
> +
> +	  If unsure, say N.
> +

WHy should this be conditional?