Linux Virtualization - May 2021 - [PATCH 2/2] x86/kexec/64: Forbid kexec when running as an SEV-ES guest

From: Joerg Roedel <jroedel at suse.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.

Cc: stable at vger.kernel.org # v5.10+
Signed-off-by: Joerg Roedel <jroedel at suse.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();
+}
-- 
2.31.1

Joerg Roedel <joro at 8bytes.org> writes:
> From: Joerg Roedel <jroedel at suse.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 at vger.kernel.org # v5.10+
> Signed-off-by: Joerg Roedel <jroedel at suse.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();
> +}