Re: [PATCH v3 11/16] powerpc/pseries/svm: Export guest SVM status to user space via sysfs

Michael Ellerman [off-list ref] writes:

Thiago Jung Bauermann [off-list ref] writes:

quoted

From: Ryan Grimm <redacted>

User space might want to know it's running in a secure VM.  It can't do
a mfmsr because mfmsr is a privileged instruction.

The solution here is to create a cpu attribute:

/sys/devices/system/cpu/svm

which will read 0 or 1 based on the S bit of the guest's CPU 0.

Why CPU 0?

If we have different CPUs running with different MSR_S then something
has gone badly wrong, no?

Yes, that would be very bad.

So can't we just read the MSR on whatever CPU the sysfs code happens to
run on.

Good point. I made the change in the patch below.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center



From 2d951305e118bf286f8e83cbf396448085186357 Mon Sep 17 00:00:00 2001
From: Ryan Grimm <redacted>
Date: Tue, 15 Jan 2019 11:56:29 -0600
Subject: [PATCH] powerpc/pseries/svm: Export guest SVM status to user space
 via sysfs

User space might want to know it's running in a secure VM.  It can't do
a mfmsr because mfmsr is a privileged instruction.

The solution here is to create a cpu attribute:

/sys/devices/system/cpu/svm

which will read 0 or 1 based on the S bit of the current CPU.

Signed-off-by: Ryan Grimm <redacted>
Signed-off-by: Thiago Jung Bauermann <redacted>
---
 arch/powerpc/kernel/sysfs.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
index e2147d7c9e72..80a676da11cb 100644
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c

@@ -19,6 +19,7 @@
 #include <asm/smp.h>
 #include <asm/pmc.h>
 #include <asm/firmware.h>
+#include <asm/svm.h>
 
 #include "cacheinfo.h"
 #include "setup.h"

@@ -715,6 +716,23 @@ static struct device_attribute pa6t_attrs[] = {
 #endif /* HAS_PPC_PMC_PA6T */
 #endif /* HAS_PPC_PMC_CLASSIC */
 
+#ifdef CONFIG_PPC_SVM
+static ssize_t show_svm(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", is_secure_guest());
+}
+static DEVICE_ATTR(svm, 0444, show_svm, NULL);
+
+static void create_svm_file(void)
+{
+	device_create_file(cpu_subsys.dev_root, &dev_attr_svm);
+}
+#else
+static void create_svm_file(void)
+{
+}
+#endif /* CONFIG_PPC_SVM */
+
 static int register_cpu_online(unsigned int cpu)
 {
 	struct cpu *c = &per_cpu(cpu_devices, cpu);

@@ -1058,6 +1076,8 @@ static int __init topology_init(void)
 	sysfs_create_dscr_default();
 #endif /* CONFIG_PPC64 */
 
+	create_svm_file();
+
 	return 0;
 }
 subsys_initcall(topology_init);