Re: [PATCH 2/6] KVM: Add KVM_PRE_FAULT_MEMORY vcpu ioctl to pre-populate guest memory

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On 4/19/2024 4:59 PM, Paolo Bonzini wrote:
From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

Add a new ioctl KVM_PRE_FAULT_MEMORY in the KVM common code. It iterates on the
memory range and calls the arch-specific function.  Add stub arch function
as a weak symbol.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Message-ID: <819322b8f25971f2b9933bfa4506e618508ad782.1712785629.git.isaku.yamahata@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
  include/linux/kvm_host.h |  5 ++++
  include/uapi/linux/kvm.h | 10 +++++++
  virt/kvm/Kconfig         |  3 ++
  virt/kvm/kvm_main.c      | 63 ++++++++++++++++++++++++++++++++++++++++
  4 files changed, 81 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8dea11701ab2..9e9943e5e37c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2478,4 +2478,9 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages
  void kvm_arch_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end);
  #endif
+#ifdef CONFIG_KVM_GENERIC_PRE_FAULT_MEMORY
+long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
+				    struct kvm_pre_fault_memory *range);
+#endif
+
  #endif
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 2190adbe3002..917d2964947d 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -917,6 +917,7 @@ struct kvm_enable_cap {
  #define KVM_CAP_MEMORY_ATTRIBUTES 233
  #define KVM_CAP_GUEST_MEMFD 234
  #define KVM_CAP_VM_TYPES 235
+#define KVM_CAP_PRE_FAULT_MEMORY 236
struct kvm_irq_routing_irqchip {
  	__u32 irqchip;
@@ -1548,4 +1549,13 @@ struct kvm_create_guest_memfd {
  	__u64 reserved[6];
  };
+#define KVM_PRE_FAULT_MEMORY _IOWR(KVMIO, 0xd5, struct kvm_pre_fault_memory)
+
+struct kvm_pre_fault_memory {
+	__u64 gpa;
+	__u64 size;
+	__u64 flags;
+	__u64 padding[5];
+};
+
  #endif /* __LINUX_KVM_H */
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 754c6c923427..b14e14cdbfb9 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -67,6 +67,9 @@ config HAVE_KVM_INVALID_WAKEUPS
  config KVM_GENERIC_DIRTYLOG_READ_PROTECT
         bool
+config KVM_GENERIC_PRE_FAULT_MEMORY
+       bool
+
  config KVM_COMPAT
         def_bool y
         depends on KVM && COMPAT && !(S390 || ARM64 || RISCV)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 38b498669ef9..51d8dbe7e93b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4379,6 +4379,55 @@ static int kvm_vcpu_ioctl_get_stats_fd(struct kvm_vcpu *vcpu)
  	return fd;
  }
+#ifdef CONFIG_KVM_GENERIC_PRE_FAULT_MEMORY
+static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
+				     struct kvm_pre_fault_memory *range)
+{
+	int idx;
+	long r;
+	u64 full_size;
+
+	if (range->flags)
+		return -EINVAL;
+
+	if (!PAGE_ALIGNED(range->gpa) ||
+	    !PAGE_ALIGNED(range->size) ||
+	    range->gpa + range->size <= range->gpa)
+		return -EINVAL;
+
+	if (!range->size)
+		return 0;

range->size equals 0 can be covered by "range->gpa + range->size <= range->gpa"

If we want to return success when size is 0 (, though I am not sure it's needed),
we need to use "range->gpa + range->size < range->gpa" instead.


+
+	vcpu_load(vcpu);
+	idx = srcu_read_lock(&vcpu->kvm->srcu);
+
+	full_size = range->size;
+	do {
+		if (signal_pending(current)) {
+			r = -EINTR;
+			break;
+		}
+
+		r = kvm_arch_vcpu_pre_fault_memory(vcpu, range);
+		if (r < 0)
+			break;
+
+		if (WARN_ON_ONCE(r == 0))
+			break;
+
+		range->size -= r;
+		range->gpa += r;
+		cond_resched();
+	} while (range->size);
+
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
+	vcpu_put(vcpu);
+
+	/* Return success if at least one page was mapped successfully.  */
+	return full_size == range->size ? r : 0;
+}
+#endif
+
  static long kvm_vcpu_ioctl(struct file *filp,
  			   unsigned int ioctl, unsigned long arg)
  {
@@ -4580,6 +4629,20 @@ static long kvm_vcpu_ioctl(struct file *filp,
  		r = kvm_vcpu_ioctl_get_stats_fd(vcpu);
  		break;
  	}
+#ifdef CONFIG_KVM_GENERIC_PRE_FAULT_MEMORY
+	case KVM_PRE_FAULT_MEMORY: {
+		struct kvm_pre_fault_memory range;
+
+		r = -EFAULT;
+		if (copy_from_user(&range, argp, sizeof(range)))
+			break;
+		r = kvm_vcpu_pre_fault_memory(vcpu, &range);
+		/* Pass back leftover range. */
+		if (copy_to_user(argp, &range, sizeof(range)))
+			r = -EFAULT;
+		break;
+	}
+#endif
  	default:
  		r = kvm_arch_vcpu_ioctl(filp, ioctl, arg);
  	}





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