Re: [PATCH v18 023/121] KVM: TDX: Make KVM_CAP_MAX_VCPUS backend specific

Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> · Sun, 4 Feb 2024 10:00:45 +0800

On 2/1/2024 2:16 PM, Yuan Yao wrote:
On Wed, Jan 24, 2024 at 09:17:15AM +0800, Binbin Wu wrote:

On 1/23/2024 7:52 AM, isaku.yamahata@xxxxxxxxx wrote:
From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>

TDX has its own limitation on the maximum number of vcpus that the guest
can accommodate.  Allow x86 kvm backend to implement its own KVM_ENABLE_CAP
handler and implement TDX backend for KVM_CAP_MAX_VCPUS.  user space VMM,
e.g. qemu, can specify its value instead of KVM_MAX_VCPUS.
For legacy VM, KVM just provides the interface to query the max_vcpus.
Why TD needs to provide a interface for userspace to set the limitation?
What's the scenario?
I think the reason is TDH.MNG.INIT needs it:

TD_PARAMS:
     MAX_VCPUS:
         offset: 16 bytes.
         type: Unsigned 16b Integer.
         size: 2.
         Description: Maximum number of VCPUs.
Thanks for explanation.

I am also wondering if this info can be passed via KVM_TDX_INIT_VM.
Because userspace is allowed to set the value no greater than
min(KVM_MAX_VCPUS, TDX_MAX_VCPUS), providing the extra cap KVM_CAP_MAX_VCPUS
doesn't make more restriction comparing to providing it in KVM_TDX_INIT_VM.


May better to clarify this in the commit yet.


Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
---
v18:
- use TDX instead of "x86, tdx" in subject
- use min(max_vcpu, TDX_MAX_VCPU) instead of
    min3(max_vcpu, KVM_MAX_VCPU, TDX_MAX_VCPU)
- make "if (KVM_MAX_VCPU) and if (TDX_MAX_VCPU)" into one if statement
---
   arch/x86/include/asm/kvm-x86-ops.h |  2 ++
   arch/x86/include/asm/kvm_host.h    |  2 ++
   arch/x86/kvm/vmx/main.c            | 22 ++++++++++++++++++++++
   arch/x86/kvm/vmx/tdx.c             | 29 +++++++++++++++++++++++++++++
   arch/x86/kvm/vmx/x86_ops.h         |  5 +++++
   arch/x86/kvm/x86.c                 |  4 ++++
   6 files changed, 64 insertions(+)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index 943b21b8b106..2f976c0f3116 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h
@@ -21,6 +21,8 @@ KVM_X86_OP(hardware_unsetup)
   KVM_X86_OP(has_emulated_msr)
   KVM_X86_OP(vcpu_after_set_cpuid)
   KVM_X86_OP(is_vm_type_supported)
+KVM_X86_OP_OPTIONAL(max_vcpus);
+KVM_X86_OP_OPTIONAL(vm_enable_cap)
   KVM_X86_OP(vm_init)
   KVM_X86_OP_OPTIONAL(vm_destroy)
   KVM_X86_OP_OPTIONAL_RET0(vcpu_precreate)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 26f4668b0273..db44a92e5659 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1602,7 +1602,9 @@ struct kvm_x86_ops {
   	void (*vcpu_after_set_cpuid)(struct kvm_vcpu *vcpu);
   	bool (*is_vm_type_supported)(unsigned long vm_type);
+	int (*max_vcpus)(struct kvm *kvm);
   	unsigned int vm_size;
+	int (*vm_enable_cap)(struct kvm *kvm, struct kvm_enable_cap *cap);
   	int (*vm_init)(struct kvm *kvm);
   	void (*vm_destroy)(struct kvm *kvm);
diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
index 50da807d7aea..4611f305a450 100644
--- a/arch/x86/kvm/vmx/main.c
+++ b/arch/x86/kvm/vmx/main.c
@@ -6,6 +6,7 @@
   #include "nested.h"
   #include "pmu.h"
   #include "tdx.h"
+#include "tdx_arch.h"
   static bool enable_tdx __ro_after_init;
   module_param_named(tdx, enable_tdx, bool, 0444);
@@ -16,6 +17,17 @@ static bool vt_is_vm_type_supported(unsigned long type)
   		(enable_tdx && tdx_is_vm_type_supported(type));
   }
+static int vt_max_vcpus(struct kvm *kvm)
+{
+	if (!kvm)
+		return KVM_MAX_VCPUS;
+
+	if (is_td(kvm))
+		return min(kvm->max_vcpus, TDX_MAX_VCPUS);
+
+	return kvm->max_vcpus;
+}
+
   static int vt_hardware_enable(void)
   {
   	int ret;
@@ -54,6 +66,14 @@ static void vt_hardware_unsetup(void)
   	vmx_hardware_unsetup();
   }
+static int vt_vm_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
+{
+	if (is_td(kvm))
+		return tdx_vm_enable_cap(kvm, cap);
+
+	return -EINVAL;
+}
+
   static int vt_vm_init(struct kvm *kvm)
   {
   	if (is_td(kvm))
@@ -91,7 +111,9 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
   	.has_emulated_msr = vmx_has_emulated_msr,
   	.is_vm_type_supported = vt_is_vm_type_supported,
+	.max_vcpus = vt_max_vcpus,
   	.vm_size = sizeof(struct kvm_vmx),
+	.vm_enable_cap = vt_vm_enable_cap,
   	.vm_init = vt_vm_init,
   	.vm_destroy = vmx_vm_destroy,
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 8c463407f8a8..876ad7895b88 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -100,6 +100,35 @@ struct tdx_info {
   /* Info about the TDX module. */
   static struct tdx_info *tdx_info;
+int tdx_vm_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
+{
+	int r;
+
+	switch (cap->cap) {
+	case KVM_CAP_MAX_VCPUS: {
+		if (cap->flags || cap->args[0] == 0)
+			return -EINVAL;
+		if (cap->args[0] > KVM_MAX_VCPUS ||
+		    cap->args[0] > TDX_MAX_VCPUS)
+			return -E2BIG;
+
+		mutex_lock(&kvm->lock);
+		if (kvm->created_vcpus)
+			r = -EBUSY;
+		else {
+			kvm->max_vcpus = cap->args[0];
+			r = 0;
+		}
+		mutex_unlock(&kvm->lock);
+		break;
+	}
+	default:
+		r = -EINVAL;
+		break;
+	}
+	return r;
+}
+
   static int tdx_get_capabilities(struct kvm_tdx_cmd *cmd)
   {
   	struct kvm_tdx_capabilities __user *user_caps;
diff --git a/arch/x86/kvm/vmx/x86_ops.h b/arch/x86/kvm/vmx/x86_ops.h
index 6e238142b1e8..3a3be66888da 100644
--- a/arch/x86/kvm/vmx/x86_ops.h
+++ b/arch/x86/kvm/vmx/x86_ops.h
@@ -139,12 +139,17 @@ int __init tdx_hardware_setup(struct kvm_x86_ops *x86_ops);
   void tdx_hardware_unsetup(void);
   bool tdx_is_vm_type_supported(unsigned long type);
+int tdx_vm_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap);
   int tdx_vm_ioctl(struct kvm *kvm, void __user *argp);
   #else
   static inline int tdx_hardware_setup(struct kvm_x86_ops *x86_ops) { return -EOPNOTSUPP; }
   static inline void tdx_hardware_unsetup(void) {}
   static inline bool tdx_is_vm_type_supported(unsigned long type) { return false; }
+static inline int tdx_vm_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
+{
+	return -EINVAL;
+};
   static inline int tdx_vm_ioctl(struct kvm *kvm, void __user *argp) { return -EOPNOTSUPP; }
   #endif
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dd3a23d56621..a1389ddb1b33 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4726,6 +4726,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
   		break;
   	case KVM_CAP_MAX_VCPUS:
   		r = KVM_MAX_VCPUS;
+		if (kvm_x86_ops.max_vcpus)
+			r = static_call(kvm_x86_max_vcpus)(kvm);
   		break;
   	case KVM_CAP_MAX_VCPU_ID:
   		r = KVM_MAX_VCPU_IDS;
@@ -6683,6 +6685,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
   		break;
   	default:
   		r = -EINVAL;
+		if (kvm_x86_ops.vm_enable_cap)
+			r = static_call(kvm_x86_vm_enable_cap)(kvm, cap);
   		break;
   	}
   	return r;









