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Date:   Thu, 6 May 2021 15:43:23 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Eric Biederman <ebiederm@xmission.com>, x86@kernel.org,
        kexec@lists.infradead.org, Joerg Roedel <jroedel@suse.de>,
        stable@vger.kernel.org, hpa@zytor.com,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jiri Slaby <jslaby@suse.cz>,
        Dan Williams <dan.j.williams@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Juergen Gross <jgross@suse.com>,
        Kees Cook <keescook@chromium.org>,
        David Rientjes <rientjes@google.com>,
        Cfir Cohen <cfir@google.com>,
        Erdem Aktas <erdemaktas@google.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mike Stunes <mstunes@vmware.com>,
        Martin Radev <martin.b.radev@gmail.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 1/2] kexec: Allow architecture code to opt-out at runtime
Message-ID: <YJQOmxx1EMUqNpNn@google.com>
References: <20210506093122.28607-1-joro@8bytes.org>
 <20210506093122.28607-2-joro@8bytes.org>
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On Thu, May 06, 2021, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> Allow a runtime opt-out of kexec support for architecture code in case
> the kernel is running in an environment where kexec is not properly
> supported yet.
> 
> This will be used on x86 when the kernel is running as an SEV-ES
> guest. SEV-ES guests need special handling for kexec to hand over all
> CPUs to the new kernel. This requires special hypervisor support and
> handling code in the guest which is not yet implemented.
> 
> Cc: stable@vger.kernel.org # v5.10+
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  kernel/kexec.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index c82c6c06f051..d03134160458 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -195,11 +195,25 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
>   * that to happen you need to do that yourself.
>   */
>  
> +bool __weak arch_kexec_supported(void)
> +{
> +	return true;
> +}
> +
>  static inline int kexec_load_check(unsigned long nr_segments,
>  				   unsigned long flags)
>  {
>  	int result;
>  
> +	/*
> +	 * The architecture may support kexec in general, but the kernel could
> +	 * run in an environment where it is not (yet) possible to execute a new
> +	 * kernel. Allow the architecture code to opt-out of kexec support when
> +	 * it is running in such an environment.
> +	 */
> +	if (!arch_kexec_supported())
> +		return -ENOSYS;

This misses kexec_file_load.  Also, is a new hook really needed?  E.g. the
SEV-ES check be shoved into machine_kexec_prepare().  The downside is that we'd
do a fair amount of work before detecting failure, but that doesn't seem hugely
problematic.

> +
>  	/* We only trust the superuser with rebooting the system. */
>  	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
>  		return -EPERM;
> -- 
> 2.31.1
>