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

[Michael Ellerman]

Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx> writes:
> Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
>> Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx> writes:
>>> From: Ryan Grimm <grimm@xxxxxxxxxxxxxxxxxx>
>>> 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.

The patch looks good. Although, it raises the question of whether it
should be an attribute of the CPU at all.

I guess there's not obviously anywhere better for it.

Still you should document the attribute in Documentation/ABI/testing/sysfs-devices-system-cpu

cheers

> From 2d951305e118bf286f8e83cbf396448085186357 Mon Sep 17 00:00:00 2001
> From: Ryan Grimm <grimm@xxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx>
> ---
>  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);