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

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Date:   Mon, 12 Aug 2019 20:21:50 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Anshuman Khandual <anshuman.linux@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christoph Hellwig <hch@....de>,
        Mike Anderson <andmike@...ux.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Ram Pai <linuxram@...ibm.com>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        Ryan Grimm <grimm@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 11/16] powerpc/pseries/svm: Export guest SVM status to user space via sysfs


Michael Ellerman <mpe@...erman.id.au> writes:

> Thiago Jung Bauermann <bauerman@...ux.ibm.com> writes:
>> From: Ryan Grimm <grimm@...ux.vnet.ibm.com>
>>
>> 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 <grimm@...ux.vnet.ibm.com>
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 <grimm@...ux.vnet.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
---
 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);