Re: [PATCH v3 3/6] x86/kexec: Disable kexec/kdump on platforms with TDX partial write erratum

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

On 6/26/2025 6:48 PM, Kai Huang wrote:
> Some early TDX-capable platforms have an erratum: A kernel partial
> write (a write transaction of less than cacheline lands at memory
> controller) to TDX private memory poisons that memory, and a subsequent
> read triggers a machine check.
>
> On those platforms, the old kernel must reset TDX private memory before
> jumping to the new kernel, otherwise the new kernel may see unexpected
> machine check.  Currently the kernel doesn't track which page is a TDX
> private page.  For simplicity just fail kexec/kdump for those platforms.
>
> Leverage the existing machine_kexec_prepare() to fail kexec/kdump by
> adding the check of the presence of the TDX erratum (which is only
> checked for if the kernel is built with TDX host support).  This rejects
> kexec/kdump when the kernel is loading the kexec/kdump kernel image.
>
> The alternative is to reject kexec/kdump when the kernel is jumping to
> the new kernel.  But for kexec this requires adding a new check (e.g.,
> arch_kexec_allowed()) in the common code to fail kernel_kexec() at early
> stage.  Kdump (crash_kexec()) needs similar check, but it's hard to
> justify because crash_kexec() is not supposed to abort.
>
> It's feasible to further relax this limitation, i.e., only fail kexec
> when TDX is actually enabled by the kernel.  But this is still a half
> measure compared to resetting TDX private memory so just do the simplest
> thing for now.
>
> The impact to userspace is the users will get an error when loading the
> kexec/kdump kernel image:
>
>    kexec_load failed: Operation not supported
>
> This might be confusing to the users, thus also print the reason in the
> dmesg:
>
>    [..] kexec: not allowed on platform with tdx_pw_mce bug.
>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> Tested-by: Farrah Chen <farrah.chen@intel.com>
> ---
>   arch/x86/kernel/machine_kexec_64.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
>
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 4519c7b75c49..d5a85d786e61 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -347,6 +347,22 @@ int machine_kexec_prepare(struct kimage *image)
>   	unsigned long reloc_end = (unsigned long)__relocate_kernel_end;
>   	int result;
>   
> +	/*
> +	 * Some early TDX-capable platforms have an erratum.  A kernel
> +	 * partial write (a write transaction of less than cacheline
> +	 * lands at memory controller) to TDX private memory poisons that
> +	 * memory, and a subsequent read triggers a machine check.
> +	 *
Nit: About the description of the erratum, maybe it's better to refer to the
comments of check_tdx_erratum() to avoid duplication. Also it gives a link to
how/when the bug is set.

Otherwise,
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>


> +	 * On those platforms the old kernel must reset TDX private
> +	 * memory before jumping to the new kernel otherwise the new
> +	 * kernel may see unexpected machine check.  For simplicity
> +	 * just fail kexec/kdump on those platforms.
> +	 */
> +	if (boot_cpu_has_bug(X86_BUG_TDX_PW_MCE)) {
> +		pr_info_once("Not allowed on platform with tdx_pw_mce bug\n");
> +		return -EOPNOTSUPP;
> +	}
> +
>   	/* Setup the identity mapped 64bit page table */
>   	result = init_pgtable(image, __pa(control_page));
>   	if (result)