Re: [PATCH v5 1/5] x86,kvm: Add MSR_KVM_GET_RNG_SEED and a matching feature bit

[Paolo Bonzini]

Il 24/07/2014 06:57, Andy Lutomirski ha scritto:
> This adds a simple interface to allow a guest to request 64 bits of
> host nonblocking entropy.  This is independent of virtio-rng for a
> couple of reasons:
> 
>  - It's intended to be usable during early boot, when a trivial
>    synchronous interface is needed.
> 
>  - virtio-rng gives blocking entropy, and making guest boot wait for
>    the host's /dev/random will cause problems.
> 
> MSR_KVM_GET_RNG_SEED is intended to provide 64 bits of best-effort
> cryptographically secure data for use as a seed.  It provides no
> guarantee that the result contains any actual entropy.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  Documentation/virtual/kvm/cpuid.txt  | 3 +++
>  arch/x86/include/uapi/asm/kvm_para.h | 2 ++
>  arch/x86/kvm/cpuid.c                 | 3 ++-
>  arch/x86/kvm/x86.c                   | 4 ++++
>  4 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 3c65feb..0ab043b 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -54,6 +54,9 @@ KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
>                                     ||       || before enabling paravirtualized
>                                     ||       || spinlock support.
>  ------------------------------------------------------------------------------
> +KVM_FEATURE_GET_RNG_SEED           ||     8 || host provides rng seed data via
> +                                   ||       || MSR_KVM_GET_RNG_SEED.
> +------------------------------------------------------------------------------
>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
>                                     ||       || per-cpu warps are expected in
>                                     ||       || kvmclock.
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 94dc8ca..e2eaf93 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -24,6 +24,7 @@
>  #define KVM_FEATURE_STEAL_TIME		5
>  #define KVM_FEATURE_PV_EOI		6
>  #define KVM_FEATURE_PV_UNHALT		7
> +#define KVM_FEATURE_GET_RNG_SEED	8
>  
>  /* The last 8 bits are used to indicate how to interpret the flags field
>   * in pvclock structure. If no bits are set, all flags are ignored.
> @@ -40,6 +41,7 @@
>  #define MSR_KVM_ASYNC_PF_EN 0x4b564d02
>  #define MSR_KVM_STEAL_TIME  0x4b564d03
>  #define MSR_KVM_PV_EOI_EN      0x4b564d04
> +#define MSR_KVM_GET_RNG_SEED 0x4b564d05
>  
>  struct kvm_steal_time {
>  	__u64 steal;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 38a0afe..40d6763 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -479,7 +479,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			     (1 << KVM_FEATURE_ASYNC_PF) |
>  			     (1 << KVM_FEATURE_PV_EOI) |
>  			     (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
> -			     (1 << KVM_FEATURE_PV_UNHALT);
> +			     (1 << KVM_FEATURE_PV_UNHALT) |
> +			     (1 << KVM_FEATURE_GET_RNG_SEED);
>  
>  		if (sched_info_on())
>  			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f644933..4e81853 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -48,6 +48,7 @@
>  #include <linux/pci.h>
>  #include <linux/timekeeper_internal.h>
>  #include <linux/pvclock_gtod.h>
> +#include <linux/random.h>
>  #include <trace/events/kvm.h>
>  
>  #define CREATE_TRACE_POINTS
> @@ -2480,6 +2481,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>  	case MSR_KVM_PV_EOI_EN:
>  		data = vcpu->arch.pv_eoi.msr_val;
>  		break;
> +	case MSR_KVM_GET_RNG_SEED:
> +		get_random_bytes(&data, sizeof(data));
> +		break;
>  	case MSR_IA32_P5_MC_ADDR:
>  	case MSR_IA32_P5_MC_TYPE:
>  	case MSR_IA32_MCG_CAP:
> 

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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