[PATCH v2 03/20] KVM: TDX: Retry locally in TDX EPT violation handler on RET_PF_RETRY

Binbin Wu
2025-02-27T01:18:59Z

From: Yan Zhao <yan.y.zhao@intel.com>

Retry locally in the TDX EPT violation handler for private memory to reduce
the chances for tdh_mem_sept_add()/tdh_mem_page_aug() to contend with
tdh_vp_enter().

TDX EPT violation installs private pages via tdh_mem_sept_add() and
tdh_mem_page_aug(). The two may have contention with tdh_vp_enter() or
TDCALLs.

Resources SHARED users EXCLUSIVE users
------------------------------------------------------------
SEPT tree tdh_mem_sept_add tdh_vp_enter(0-step mitigation)
tdh_mem_page_aug
------------------------------------------------------------
SEPT entry tdh_mem_sept_add (Host lock)
tdh_mem_page_aug (Host lock)
tdg_mem_page_accept (Guest lock)
tdg_mem_page_attr_rd (Guest lock)
tdg_mem_page_attr_wr (Guest lock)

Though the contention between tdh_mem_sept_add()/tdh_mem_page_aug() and
TDCALLs may be removed in future TDX module, their contention with
tdh_vp_enter() due to 0-step mitigation still persists.

The TDX module may trigger 0-step mitigation in SEAMCALL TDH.VP.ENTER,
which works as follows:
0. Each TDH.VP.ENTER records the guest RIP on TD entry.
1. When the TDX module encounters a VM exit with reason EPT_VIOLATION, it
checks if the guest RIP is the same as last guest RIP on TD entry.
-if yes, it means the EPT violation is caused by the same instruction
that caused the last VM exit.
Then, the TDX module increases the guest RIP no-progress count.
When the count increases from 0 to the threshold (currently 6),
the TDX module records the faulting GPA into a
last_epf_gpa_list.
-if no, it means the guest RIP has made progress.
So, the TDX module resets the RIP no-progress count and the
last_epf_gpa_list.
2. On the next TDH.VP.ENTER, the TDX module (after saving the guest RIP on
TD entry) checks if the last_epf_gpa_list is empty.
-if yes, TD entry continues without acquiring the lock on the SEPT tree.
-if no, it triggers the 0-step mitigation by acquiring the exclusive
lock on SEPT tree, walking the EPT tree to check if all page
faults caused by the GPAs in the last_epf_gpa_list have been
resolved before continuing TD entry.

Since KVM TDP MMU usually re-enters guest whenever it exits to userspace
(e.g. for KVM_EXIT_MEMORY_FAULT) or encounters a BUSY, it is possible for a
tdh_vp_enter() to be called more than the threshold count before a page
fault is addressed, triggering contention when tdh_vp_enter() attempts to
acquire exclusive lock on SEPT tree.

Retry locally in TDX EPT violation handler to reduce the count of invoking
tdh_vp_enter(), hence reducing the possibility of its contention with
tdh_mem_sept_add()/tdh_mem_page_aug(). However, the 0-step mitigation and
the contention are still not eliminated due to KVM_EXIT_MEMORY_FAULT,
signals/interrupts, and cases when one instruction faults more GFNs than
the threshold count.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
Changes from "KVM: TDX SEPT SEAMCALL retry" series [1]
- Use kvm_vcpu_has_events() to replace "pi_has_pending_interrupt(vcpu) ||
kvm_test_request(KVM_REQ_NMI, vcpu) || vcpu->arch.nmi_pending)" [2].
(Sean)
- Update code comment to explain why break out for false positive of
interrupt injection is fine (Sean).

[1]: https://lore.kernel.org/all/20250113021218.18922-1-yan.y.zhao@intel.com
[2]: https://lore.kernel.org/all/Z4rIGv4E7Jdmhl8P@google.com
---
arch/x86/kvm/vmx/tdx.c | 57 +++++++++++++++++++++++++++++++++++++++++-
1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b8701e343e80..6fa6f7e13e15 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -1698,6 +1698,8 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
{
unsigned long exit_qual;
gpa_t gpa = to_tdx(vcpu)->exit_gpa;
+ bool local_retry = false;
+ int ret;

if (vt_is_tdx_private_gpa(vcpu->kvm, gpa)) {
if (tdx_is_sept_violation_unexpected_pending(vcpu)) {
@@ -1716,6 +1718,9 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
* due to aliasing a single HPA to multiple GPAs.
*/
exit_qual = EPT_VIOLATION_ACC_WRITE;
+
+ /* Only private GPA triggers zero-step mitigation */
+ local_retry = true;
} else {
exit_qual = vmx_get_exit_qual(vcpu);
/*
@@ -1728,7 +1733,57 @@ static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
}

trace_kvm_page_fault(vcpu, gpa, exit_qual);
- return __vmx_handle_ept_violation(vcpu, gpa, exit_qual);
+
+ /*
+ * To minimize TDH.VP.ENTER invocations, retry locally for private GPA
+ * mapping in TDX.
+ *
+ * KVM may return RET_PF_RETRY for private GPA due to
+ * - contentions when atomically updating SPTEs of the mirror page table
+ * - in-progress GFN invalidation or memslot removal.
+ * - TDX_OPERAND_BUSY error from TDH.MEM.PAGE.AUG or TDH.MEM.SEPT.ADD,
+ * caused by contentions with TDH.VP.ENTER (with zero-step mitigation)
+ * or certain TDCALLs.
+ *
+ * If TDH.VP.ENTER is invoked more times than the threshold set by the
+ * TDX module before KVM resolves the private GPA mapping, the TDX
+ * module will activate zero-step mitigation during TDH.VP.ENTER. This
+ * process acquires an SEPT tree lock in the TDX module, leading to
+ * further contentions with TDH.MEM.PAGE.AUG or TDH.MEM.SEPT.ADD
+ * operations on other vCPUs.
+ *
+ * Breaking out of local retries for kvm_vcpu_has_events() is for
+ * interrupt injection. kvm_vcpu_has_events() should not see pending
+ * events for TDX. Since KVM can't determine if IRQs (or NMIs) are
+ * blocked by TDs, false positives are inevitable i.e., KVM may re-enter
+ * the guest even if the IRQ/NMI can't be delivered.
+ *
+ * Note: even without breaking out of local retries, zero-step
+ * mitigation may still occur due to
+ * - invoking of TDH.VP.ENTER after KVM_EXIT_MEMORY_FAULT,
+ * - a single RIP causing EPT violations for more GFNs than the
+ * threshold count.
+ * This is safe, as triggering zero-step mitigation only introduces
+ * contentions to page installation SEAMCALLs on other vCPUs, which will
+ * handle retries locally in their EPT violation handlers.
+ */
+ while (1) {
+ ret = __vmx_handle_ept_violation(vcpu, gpa, exit_qual);
+
+ if (ret != RET_PF_RETRY || !local_retry)
+ break;
+
+ if (kvm_vcpu_has_events(vcpu) || signal_pending(current))
+ break;
+
+ if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu)) {
+ ret = -EIO;
+ break;
+ }
+
+ cond_resched();
+ }
+ return ret;
}

int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
--
2.46.0