Date:   Fri, 2 Apr 2021 16:25:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nathan Tempelman <natet@...gle.com>
Cc:     thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, srutherford@...gle.com,
        seanjc@...gle.com, rientjes@...gle.com, brijesh.singh@....com,
        Ashish.Kalra@....com, dovmurik@...ux.vnet.ibm.com,
        lersek@...hat.com, jejb@...ux.ibm.com, frankeh@...ibm.com
Subject: Re: [RFC v2] KVM: x86: Support KVM VMs sharing SEV context

On 16/03/21 02:40, Nathan Tempelman wrote:
> Add a capability for userspace to mirror SEV encryption context from
> one vm to another. On our side, this is intended to support a
> Migration Helper vCPU, but it can also be used generically to support
> other in-guest workloads scheduled by the host. The intention is for
> the primary guest and the mirror to have nearly identical memslots.
> 
> The primary benefits of this are that:
> 1) The VMs do not share KVM contexts (think APIC/MSRs/etc), so they
> can't accidentally clobber each other.
> 2) The VMs can have different memory-views, which is necessary for post-copy
> migration (the migration vCPUs on the target need to read and write to
> pages, when the primary guest would VMEXIT).
> 
> This does not change the threat model for AMD SEV. Any memory involved
> is still owned by the primary guest and its initial state is still
> attested to through the normal SEV_LAUNCH_* flows. If userspace wanted
> to circumvent SEV, they could achieve the same effect by simply attaching
> a vCPU to the primary VM.
> This patch deliberately leaves userspace in charge of the memslots for the
> mirror, as it already has the power to mess with them in the primary guest.
> 
> This patch does not support SEV-ES (much less SNP), as it does not
> handle handing off attested VMSAs to the mirror.
> 
> For additional context, we need a Migration Helper because SEV PSP migration
> is far too slow for our live migration on its own. Using an in-guest
> migrator lets us speed this up significantly.
> 
> Signed-off-by: Nathan Tempelman <natet@...gle.com>
> ---
>   Documentation/virt/kvm/api.rst  | 17 +++++++
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/svm/sev.c          | 88 +++++++++++++++++++++++++++++++++
>   arch/x86/kvm/svm/svm.c          |  2 +
>   arch/x86/kvm/svm/svm.h          |  2 +
>   arch/x86/kvm/x86.c              |  7 ++-
>   include/linux/kvm_host.h        |  1 +
>   include/uapi/linux/kvm.h        |  1 +
>   virt/kvm/kvm_main.c             |  6 +++
>   9 files changed, 124 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 482508ec7cc4..332ba8b5b6f4 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6213,6 +6213,23 @@ the bus lock vm exit can be preempted by a higher priority VM exit, the exit
>   notifications to userspace can be KVM_EXIT_BUS_LOCK or other reasons.
>   KVM_RUN_BUS_LOCK flag is used to distinguish between them.
>   
> +7.23 KVM_CAP_VM_COPY_ENC_CONTEXT_FROM
> +-------------------------------------
> +
> +Architectures: x86 SEV enabled
> +Type: vm
> +Parameters: args[0] is the fd of the source vm
> +Returns: 0 on success; ENOTTY on error
> +
> +This capability enables userspace to copy encryption context from the vm
> +indicated by the fd to the vm this is called on.
> +
> +This is intended to support in-guest workloads scheduled by the host. This
> +allows the in-guest workload to maintain its own NPTs and keeps the two vms
> +from accidentally clobbering each other with interrupts and the like (separate
> +APIC/MSRs/etc).
> +
> +
>   8. Other capabilities.
>   ======================
>   
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 84499aad01a4..46df415a8e91 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1334,6 +1334,7 @@ struct kvm_x86_ops {
>   	int (*mem_enc_op)(struct kvm *kvm, void __user *argp);
>   	int (*mem_enc_reg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
>   	int (*mem_enc_unreg_region)(struct kvm *kvm, struct kvm_enc_region *argp);
> +	int (*vm_copy_enc_context_from)(struct kvm *kvm, unsigned int source_fd);
>   
>   	int (*get_msr_feature)(struct kvm_msr_entry *entry);
>   
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 874ea309279f..b2c90c67a0d9 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -66,6 +66,11 @@ static int sev_flush_asids(void)
>   	return ret;
>   }
>   
> +static inline bool is_mirroring_enc_context(struct kvm *kvm)
> +{
> +	return to_kvm_svm(kvm)->sev_info.enc_context_owner;
> +}
> +
>   /* Must be called with the sev_bitmap_lock held */
>   static bool __sev_recycle_asids(int min_asid, int max_asid)
>   {
> @@ -1124,6 +1129,10 @@ int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
>   	if (copy_from_user(&sev_cmd, argp, sizeof(struct kvm_sev_cmd)))
>   		return -EFAULT;
>   
> +	/* enc_context_owner handles all memory enc operations */
> +	if (is_mirroring_enc_context(kvm))
> +		return -ENOTTY;
> +
>   	mutex_lock(&kvm->lock);
>   
>   	switch (sev_cmd.id) {
> @@ -1186,6 +1195,10 @@ int svm_register_enc_region(struct kvm *kvm,
>   	if (!sev_guest(kvm))
>   		return -ENOTTY;
>   
> +	/* If kvm is mirroring encryption context it isn't responsible for it */
> +	if (is_mirroring_enc_context(kvm))
> +		return -ENOTTY;
> +
>   	if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
>   		return -EINVAL;
>   
> @@ -1252,6 +1265,10 @@ int svm_unregister_enc_region(struct kvm *kvm,
>   	struct enc_region *region;
>   	int ret;
>   
> +	/* If kvm is mirroring encryption context it isn't responsible for it */
> +	if (is_mirroring_enc_context(kvm))
> +		return -ENOTTY;
> +
>   	mutex_lock(&kvm->lock);
>   
>   	if (!sev_guest(kvm)) {
> @@ -1282,6 +1299,71 @@ int svm_unregister_enc_region(struct kvm *kvm,
>   	return ret;
>   }
>   
> +int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
> +{
> +	struct file *source_kvm_file;
> +	struct kvm *source_kvm;
> +	struct kvm_sev_info *mirror_sev;
> +	unsigned int asid;
> +	int ret;
> +
> +	source_kvm_file = fget(source_fd);
> +	if (!file_is_kvm(source_kvm_file)) {
> +		ret = -EBADF;
> +		goto e_source_put;
> +	}
> +
> +	source_kvm = source_kvm_file->private_data;
> +	mutex_lock(&source_kvm->lock);
> +
> +	if (!sev_guest(source_kvm)) {
> +		ret = -ENOTTY;
> +		goto e_source_unlock;
> +	}
> +
> +	/* Mirrors of mirrors should work, but let's not get silly */
> +	if (is_mirroring_enc_context(source_kvm) || source_kvm == kvm) {
> +		ret = -ENOTTY;
> +		goto e_source_unlock;
> +	}
> +
> +	asid = to_kvm_svm(source_kvm)->sev_info.asid;
> +
> +	/*
> +	 * The mirror kvm holds an enc_context_owner ref so its asid can't
> +	 * disappear until we're done with it
> +	 */
> +	kvm_get_kvm(source_kvm);
> +
> +	fput(source_kvm_file);
> +	mutex_unlock(&source_kvm->lock);
> +	mutex_lock(&kvm->lock);
> +
> +	if (sev_guest(kvm)) {
> +		ret = -ENOTTY;
> +		goto e_mirror_unlock;
> +	}
> +
> +	/* Set enc_context_owner and copy its encryption context over */
> +	mirror_sev = &to_kvm_svm(kvm)->sev_info;
> +	mirror_sev->enc_context_owner = source_kvm;
> +	mirror_sev->asid = asid;
> +	mirror_sev->active = true;
> +
> +	mutex_unlock(&kvm->lock);
> +	return 0;
> +
> +e_mirror_unlock:
> +	mutex_unlock(&kvm->lock);
> +	kvm_put_kvm(source_kvm);
> +	return ret;
> +e_source_unlock:
> +	mutex_unlock(&source_kvm->lock);
> +e_source_put:
> +	fput(source_kvm_file);
> +	return ret;
> +}
> +
>   void sev_vm_destroy(struct kvm *kvm)
>   {
>   	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> @@ -1293,6 +1375,12 @@ void sev_vm_destroy(struct kvm *kvm)
>   
>   	mutex_lock(&kvm->lock);
>   
> +	/* If this is a mirror_kvm release the enc_context_owner and skip sev cleanup */
> +	if (is_mirroring_enc_context(kvm)) {
> +		kvm_put_kvm(sev->enc_context_owner);
> +		return;
> +	}
> +
>   	/*
>   	 * Ensure that all guest tagged cache entries are flushed before
>   	 * releasing the pages back to the system for use. CLFLUSH will
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 42d4710074a6..9ffb2bcf5389 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4608,6 +4608,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>   	.mem_enc_reg_region = svm_register_enc_region,
>   	.mem_enc_unreg_region = svm_unregister_enc_region,
>   
> +	.vm_copy_enc_context_from = svm_vm_copy_asid_from,
> +
>   	.can_emulate_instruction = svm_can_emulate_instruction,
>   
>   	.apic_init_signal_blocked = svm_apic_init_signal_blocked,
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 39e071fdab0c..779009839f6a 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -65,6 +65,7 @@ struct kvm_sev_info {
>   	unsigned long pages_locked; /* Number of pages locked */
>   	struct list_head regions_list;  /* List of registered regions */
>   	u64 ap_jump_table;	/* SEV-ES AP Jump Table address */
> +	struct kvm *enc_context_owner; /* Owner of copied encryption context */
>   };
>   
>   struct kvm_svm {
> @@ -561,6 +562,7 @@ int svm_register_enc_region(struct kvm *kvm,
>   			    struct kvm_enc_region *range);
>   int svm_unregister_enc_region(struct kvm *kvm,
>   			      struct kvm_enc_region *range);
> +int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd);
>   void pre_sev_run(struct vcpu_svm *svm, int cpu);
>   void __init sev_hardware_setup(void);
>   void sev_hardware_teardown(void);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3fa140383f5d..343cb05c2a24 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3753,6 +3753,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	case KVM_CAP_X86_USER_SPACE_MSR:
>   	case KVM_CAP_X86_MSR_FILTER:
>   	case KVM_CAP_ENFORCE_PV_FEATURE_CPUID:
> +	case KVM_CAP_VM_COPY_ENC_CONTEXT_FROM:
>   		r = 1;
>   		break;
>   	case KVM_CAP_XEN_HVM:
> @@ -4649,7 +4650,6 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
>   			kvm_update_pv_runtime(vcpu);
>   
>   		return 0;
> -
>   	default:
>   		return -EINVAL;
>   	}
> @@ -5321,6 +5321,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   			kvm->arch.bus_lock_detection_enabled = true;
>   		r = 0;
>   		break;
> +	case KVM_CAP_VM_COPY_ENC_CONTEXT_FROM:
> +		r = -ENOTTY;
> +		if (kvm_x86_ops.vm_copy_enc_context_from)
> +			r = kvm_x86_ops.vm_copy_enc_context_from(kvm, cap->args[0]);
> +		return r;
>   	default:
>   		r = -EINVAL;
>   		break;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e126ebda36d0..dc5a81115df7 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -637,6 +637,7 @@ void kvm_exit(void);
>   
>   void kvm_get_kvm(struct kvm *kvm);
>   void kvm_put_kvm(struct kvm *kvm);
> +bool file_is_kvm(struct file *file);
>   void kvm_put_kvm_no_destroy(struct kvm *kvm);
>   
>   static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 63f8f6e95648..9dc00f9baf54 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1077,6 +1077,7 @@ struct kvm_ppc_resize_hpt {
>   #define KVM_CAP_SYS_HYPERV_CPUID 191
>   #define KVM_CAP_DIRTY_LOG_RING 192
>   #define KVM_CAP_X86_BUS_LOCK_EXIT 193
> +#define KVM_CAP_VM_COPY_ENC_CONTEXT_FROM 194
>   
>   #ifdef KVM_CAP_IRQ_ROUTING
>   
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 001b9de4e727..5baf82b01e0c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4041,6 +4041,12 @@ static struct file_operations kvm_vm_fops = {
>   	KVM_COMPAT(kvm_vm_compat_ioctl),
>   };
>   
> +bool file_is_kvm(struct file *file)
> +{
> +	return file && file->f_op == &kvm_vm_fops;
> +}
> +EXPORT_SYMBOL_GPL(file_is_kvm);
> +
>   static int kvm_dev_ioctl_create_vm(unsigned long type)
>   {
>   	int r;
> 

Queued, thanks.

Paolo

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