[RFC PATCH v2.1] x86/sev-es: Handle NMI State

Joerg Roedel
2020-03-20T13:17:17Z

On Thu, Mar 19, 2020 at 08:35:59AM -0700, Andy Lutomirski wrote:
> 1. Just put the NMI unmask in do_nmi(). The kernel *already* knows
> how to handle running do_nmi() with NMIs unmasked. This is much, much
> simpler than your code.

Okay, attached is the updated patch which implements this approach. I
tested it in an SEV-ES guest with 'perf top' running for a little more
than 30 minutes and all looked good. I also removed the dead code from
the patch.

From ec3b021c5d9130fd66e00d823c4fabc675c4b49e Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroedel@suse.de>
Date: Tue, 28 Jan 2020 17:31:05 +0100
Subject: [PATCH] x86/sev-es: Handle NMI State

When running under SEV-ES the kernel has to tell the hypervisor when to
open the NMI window again after an NMI was injected. This is done with
an NMI-complete message to the hypervisor.

Add code to the kernels NMI handler to send this message right at the
beginning of do_nmi(). This always allows nesting NMIs.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/include/asm/sev-es.h | 2 ++
arch/x86/include/uapi/asm/svm.h | 1 +
arch/x86/kernel/nmi.c | 8 ++++++++
arch/x86/kernel/sev-es.c | 18 ++++++++++++++++++
4 files changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/sev-es.h b/arch/x86/include/asm/sev-es.h
index 63acf50e6280..441ec1ba2cc7 100644
--- a/arch/x86/include/asm/sev-es.h
+++ b/arch/x86/include/asm/sev-es.h
@@ -82,11 +82,13 @@ struct real_mode_header;

#ifdef CONFIG_AMD_MEM_ENCRYPT
int sev_es_setup_ap_jump_table(struct real_mode_header *rmh);
+void sev_es_nmi_complete(void);
#else /* CONFIG_AMD_MEM_ENCRYPT */
static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh)
{
return 0;
}
+static inline void sev_es_nmi_complete(void) { }
#endif /* CONFIG_AMD_MEM_ENCRYPT*/

#endif
diff --git a/arch/x86/include/uapi/asm/svm.h b/arch/x86/include/uapi/asm/svm.h
index 20a05839dd9a..0f837339db66 100644
--- a/arch/x86/include/uapi/asm/svm.h
+++ b/arch/x86/include/uapi/asm/svm.h
@@ -84,6 +84,7 @@
/* SEV-ES software-defined VMGEXIT events */
#define SVM_VMGEXIT_MMIO_READ 0x80000001
#define SVM_VMGEXIT_MMIO_WRITE 0x80000002
+#define SVM_VMGEXIT_NMI_COMPLETE 0x80000003
#define SVM_VMGEXIT_AP_HLT_LOOP 0x80000004
#define SVM_VMGEXIT_AP_JUMP_TABLE 0x80000005
#define SVM_VMGEXIT_SET_AP_JUMP_TABLE 0
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 54c21d6abd5a..fc872a7e0ed1 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -37,6 +37,7 @@
#include <asm/reboot.h>
#include <asm/cache.h>
#include <asm/nospec-branch.h>
+#include <asm/sev-es.h>

#define CREATE_TRACE_POINTS
#include <trace/events/nmi.h>
@@ -510,6 +511,13 @@ NOKPROBE_SYMBOL(is_debug_stack);
dotraplinkage notrace void
do_nmi(struct pt_regs *regs, long error_code)
{
+ /*
+ * Re-enable NMIs right here when running as an SEV-ES guest. This might
+ * cause nested NMIs, but those can be handled safely.
+ */
+ if (sev_es_active())
+ sev_es_nmi_complete();
+
if (IS_ENABLED(CONFIG_SMP) && cpu_is_offline(smp_processor_id()))
return;

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 3c22f256645e..a7e2739771e7 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -270,6 +270,24 @@ static phys_addr_t vc_slow_virt_to_phys(struct ghcb *ghcb, long vaddr)
/* Include code shared with pre-decompression boot stage */
#include "sev-es-shared.c"

+void sev_es_nmi_complete(void)
+{
+ struct ghcb_state state;
+ struct ghcb *ghcb;
+
+ ghcb = sev_es_get_ghcb(&state);
+
+ vc_ghcb_invalidate(ghcb);
+ ghcb_set_sw_exit_code(ghcb, SVM_VMGEXIT_NMI_COMPLETE);
+ ghcb_set_sw_exit_info_1(ghcb, 0);
+ ghcb_set_sw_exit_info_2(ghcb, 0);
+
+ sev_es_wr_ghcb_msr(__pa(ghcb));
+ VMGEXIT();
+
+ sev_es_put_ghcb(&state);
+}
+
static u64 sev_es_get_jump_table_addr(void)
{
struct ghcb_state state;
--
2.16.4