[PATCH 07/25] KVM: TDX: Add helper functions to allocate/free TDX private host key id

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: Isaku Yamahata <isaku.yamahata@intel.com>

Add helper functions to allocate/free TDX private host key id (HKID).

The memory controller encrypts TDX memory with the assigned HKIDs. Each TDX
guest must be protected by its own unique TDX HKID.

The HW has a fixed set of these HKID keys. Out of those, some are set aside
for use by for other TDX components, but most are saved for guest use. The
code that does this partitioning, records the range chosen to be available
for guest use in the tdx_guest_keyid_start and tdx_nr_guest_keyids
variables.

Use this range of HKIDs reserved for guest use with the kernel's IDA
allocator library helper to create a mini TDX HKID allocator that can be
called when setting up a TD. This way it can have an exclusive HKID, as is
required. This allocator will be used in future changes.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
uAPI breakout v1:
 - Update the commit message
 - Delete stale comment on global hkdi
 - Deleted WARN_ON_ONCE() as it doesn't seemed very usefull

v19:
 - Removed stale comment in tdx_guest_keyid_alloc() by Binbin
 - Update sanity check in tdx_guest_keyid_free() by Binbin

v18:
 - Moved the functions to kvm tdx from arch/x86/virt/vmx/tdx/
 - Drop exporting symbols as the host tdx does.
---
 arch/x86/kvm/vmx/tdx.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index dbcc1ed80efa..b1c885ce8c9c 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -14,6 +14,21 @@ static enum cpuhp_state tdx_cpuhp_state;
 
 static const struct tdx_sysinfo *tdx_sysinfo;
 
+/* TDX KeyID pool */
+static DEFINE_IDA(tdx_guest_keyid_pool);
+
+static int __used tdx_guest_keyid_alloc(void)
+{
+	return ida_alloc_range(&tdx_guest_keyid_pool, tdx_guest_keyid_start,
+			       tdx_guest_keyid_start + tdx_nr_guest_keyids - 1,
+			       GFP_KERNEL);
+}
+
+static void __used tdx_guest_keyid_free(int keyid)
+{
+	ida_free(&tdx_guest_keyid_pool, keyid);
+}
+
 static int tdx_online_cpu(unsigned int cpu)
 {
 	unsigned long flags;
-- 
2.34.1