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 23:03:16 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     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>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Subject: Re: [PATCH v3 11/16] powerpc/pseries/svm: Export guest SVM status to user space via sysfs

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?

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

cheers

> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> index e2147d7c9e72..f7100ab77d29 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 +716,32 @@ static struct device_attribute pa6t_attrs[] = {
>  #endif /* HAS_PPC_PMC_PA6T */
>  #endif /* HAS_PPC_PMC_CLASSIC */
>  
> +#ifdef CONFIG_PPC_SVM
> +static void get_svm(void *val)
> +{
> +	u32 *value = val;
> +
> +	*value = is_secure_guest();
> +}
> +
> +static ssize_t show_svm(struct device *dev, struct device_attribute *attr, char *buf)
> +{
> +	u32 val;
> +	smp_call_function_single(0, get_svm, &val, 1);
> +	return sprintf(buf, "%u\n", val);
> +}
> +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 +1085,8 @@ static int __init topology_init(void)
>  	sysfs_create_dscr_default();
>  #endif /* CONFIG_PPC64 */
>  
> +	create_svm_file();
> +
>  	return 0;
>  }
>  subsys_initcall(topology_init);