linux-kernel - Re: [RFC PATCH] x86/virt/tdx: Disable KEXEC in the presence of TDX

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Date: Thu, 18 Jan 2024 21:31:24 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "nik.borisov@...e.com" <nik.borisov@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"x86@...nel.org" <x86@...nel.org>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>
Subject: Re: [RFC PATCH] x86/virt/tdx: Disable KEXEC in the presence of TDX

On Thu, 2024-01-18 at 18:01 +0200, Nikolay Borisov wrote:
> TDX doesn't currently support kexec so disable the latter when TDX
> is detected at boot time. If kexec must absolutely be supported then
> TDX can be disabled from the bios.
> 
> Making this decision at run time rather than as a compile time option
> allows distribution kernels to have both enabled and delegate to the
> user whether they want to use TDX/Kexec.
> 
> Signed-off-by: Nikolay Borisov <nik.borisov@...e.com>

Hi Nikolay,

Thanks for the patch.

I am working in progress on patches to make kexec() work with TDX.  I plan to
send them out soon (e.g., next week), but I think perhaps your patch is
reasonable before that is merged.

Acked-by: Kai Huang <kai.huang@...el.com>

> ---
> 
> Sending as RFC as I don't know the reason why initially it was decided to
> enforce the incompaibility between kexec/tdx at build time rather than runtime.
> 
>  arch/x86/Kconfig            | 1 -
>  arch/x86/virt/vmx/tdx/tdx.c | 6 ++++++
>  include/linux/kexec.h       | 1 +
>  kernel/kexec_core.c         | 5 +++++
>  4 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 92c03cb99b3e..6c2b620b40d8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1973,7 +1973,6 @@ config INTEL_TDX_HOST
>  	depends on X86_X2APIC
>  	select ARCH_KEEP_MEMBLOCK
>  	depends on CONTIG_ALLOC
> -	depends on !KEXEC_CORE
>  	depends on X86_MCE
>  	help
>  	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index 4d6826a76f78..270090bd7b5e 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -20,6 +20,7 @@
>  #include <linux/memblock.h>
>  #include <linux/memory.h>
>  #include <linux/minmax.h>
> +#include <linux/kexec.h>
>  #include <linux/sizes.h>
>  #include <linux/pfn.h>
>  #include <linux/align.h>
> @@ -1473,6 +1474,11 @@ void __init tdx_init(void)
>  		return;
>  	}
> 
> +#ifdef CONFIG_KEXEC_CORE
> +	kexec_disable();
> +	pr_info("Disable Kexec. Turn off TDX in the BIOS to use KEXEC.\n");
> +#endif
> +
>  #if defined(CONFIG_ACPI) && defined(CONFIG_SUSPEND)
>  	pr_info("Disable ACPI S3. Turn off TDX in the BIOS to use ACPI S3.\n");
>  	acpi_suspend_lowlevel = NULL;
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 8227455192b7..c434c8569242 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -500,6 +500,7 @@ static inline int crash_hotplug_memory_support(void) { return 0; }
>  static inline unsigned int crash_get_elfcorehdr_size(void) { return 0; }
>  #endif
> 
> +extern void kexec_disable(void);
>  #else /* !CONFIG_KEXEC_CORE */
>  struct pt_regs;
>  struct task_struct;
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index be5642a4ec49..8d3a2a04ff4d 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -927,6 +927,11 @@ struct kimage *kexec_image;
>  struct kimage *kexec_crash_image;
>  static int kexec_load_disabled;
> 
> +void kexec_disable(void)
> +{
> +	kexec_load_disabled = 1;
> +}
> +
>  #ifdef CONFIG_SYSCTL
>  static int kexec_limit_handler(struct ctl_table *table, int write,
>  			       void *buffer, size_t *lenp, loff_t *ppos)
> --
> 2.34.1
> 
>