linux-kernel - Re: [PATCH 2/2] x86/kexec/64: Forbid kexec when running as an SEV-ES guest

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Message-ID: <m17dkb4v4k.fsf@fess.ebiederm.org>
Date:   Thu, 06 May 2021 12:42:03 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, kexec@...ts.infradead.org,
        Joerg Roedel <jroedel@...e.de>, stable@...r.kernel.org,
        hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/kexec/64: Forbid kexec when running as an SEV-ES guest

Joerg Roedel <joro@...tes.org> writes:

> From: Joerg Roedel <jroedel@...e.de>
>
> For now, kexec is not supported when running as an SEV-ES guest. Doing
> so requires additional hypervisor support and special code to hand
> over the CPUs to the new kernel in a safe way.
>
> Until this is implemented, do not support kexec in SEV-ES guests.

I don't understand this.

Fundamentally kexec is about doing things more or less inspite of
what the firmware is doing.

I don't have any idea what a SEV-ES is.  But the normal x86 boot doesn't
do anything special.  Is cross cpu IPI emulation buggy?

If this is a move in your face hypervisor like Xen is sometimes I can
see perhaps needing a little bit of different work during bootup.
Perhaps handing back a cpu on system shutdown and asking for more cpus
on system boot up.

What is the actual problem you are trying to avoid?

And yes for a temporary hack the suggestion of putting code into
machine_kexec_prepare seems much more reasonable so we don't have to
carry special case infrastructure for the forseeable future.

Eric


> Cc: stable@...r.kernel.org # v5.10+
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/kernel/machine_kexec_64.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index c078b0d3ab0e..f902cc9cc634 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -620,3 +620,11 @@ void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages)
>  	 */
>  	set_memory_encrypted((unsigned long)vaddr, pages);
>  }
> +
> +/*
> + * Kexec is not supported in SEV-ES guests yet
> + */
> +bool arch_kexec_supported(void)
> +{
> +	return !sev_es_active();
> +}