Re: [PATCH 5/7] KVM: x86: Implement KVM_HYPERV_SET_TLB_FLUSH_INHIBIT

On 10.10.24 10:57, Vitaly Kuznetsov wrote:

Nikolas Wipper nikwip@amazon.de writes:

...

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 7571ac578884..ab3a9beb61a2 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -698,6 +698,8 @@ struct kvm_vcpu_hv {

bool suspended; int waiting_on;

• int tlb_flush_inhibit;

This is basically boolean, right? And we only make it 'int' to be able to store 'u8' from the ioctl? This doesn't look very clean. Do you envision anything but '1'/'0' in 'inhibit'? If not, maybe we can just make it a flag (and e.g. extend 'flags' to be u32/u64)? This way we can convert 'tlb_flush_inhibit' to a normal bool.

Yes, inhibit would always be binary, so incorporating it into the flags sounds reasonable. Even with the current API, this could just be a bool (tlb_flush_inhibit = inhibit == 1;)

...

};

struct kvm_hypervisor_cpuid { diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c index e68fbc0c7fc1..40ea8340838f 100644 --- a/arch/x86/kvm/hyperv.c +++ b/arch/x86/kvm/hyperv.c @@ -2137,6 +2137,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc) bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);

kvm_for_each_vcpu(i, v, kvm) {

• 	if (READ_ONCE(v->arch.hyperv->tlb_flush_inhibit))
  

• 		goto ret_suspend;
  

• __set_bit(i, vcpu_mask);
  

  } } else if (!is_guest_mode(vcpu)) {

@@ -2148,6 +2151,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc) __clear_bit(i, vcpu_mask); continue; }

• 	if (READ_ONCE(v->arch.hyperv->tlb_flush_inhibit))
  

• 		goto ret_suspend;
  

  } } else { struct kvm_vcpu_hv *hv_v;

@@ -2175,6 +2181,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc) sparse_banks)) continue;

• 	if (READ_ONCE(v->arch.hyperv->tlb_flush_inhibit))
  

• 		goto ret_suspend;
  

These READ_ONCEs make me think I misunderstand something here, please bear with me :-).

Like we're trying to protect against 'tlb_flush_inhibit' being read somewhere in the beginning of the function and want to generate real memory accesses. But what happens if tlb_flush_inhibit changes right _after_ we checked it here and _before_ we actuall do kvm_make_vcpus_request_mask()? Wouldn't it be a problem? In case it would, I think we need to reverse the order: do kvm_make_vcpus_request_mask() anyway and after it go through vcpu_mask checking whether any of the affected vCPUs has 'tlb_flush_inhibit' and if it does, suspend the caller.

The case you're describing is prevented through SRCU synchronisation in the ioctl. The hypercall actually holds a read side critical section during the whole of its execution, so when tlb_flush_inhibit changes after we read it, the ioctl would wait for the flushes to complete:

vCPU 0 | vCPU 1 -------------------------+------------------------ | hypercall enter | srcu_read_lock() ioctl enter | | tlb_flush_inhibit read tlb_flush_inhibit write | synchronize_srcu() start | | TLB flush reqs send | srcu_read_unlock() synchronize_srcu() end | ioctl exit |

...

	__set_bit(i, vcpu_mask);
}

} @@ -2193,6 +2202,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc) /* We always do full TLB flush, set 'Reps completed' = 'Rep Count' */ return (u64)HV_STATUS_SUCCESS | ((u64)hc->rep_cnt << HV_HYPERCALL_REP_COMP_OFFSET); +ret_suspend:

• kvm_hv_vcpu_suspend_tlb_flush(vcpu, v->vcpu_id);
• return -EBUSY;

}

static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector, @@ -2380,6 +2392,13 @@ static int kvm_hv_hypercall_complete(struct kvm_vcpu *vcpu, u64 result) u32 tlb_lock_count = 0; int ret;

• /*

• * Reached when the hyper-call resulted in a suspension of the vCPU.
  

• * The instruction will be re-tried once the vCPU is unsuspended.
  

• */
  

• if (kvm_hv_vcpu_suspended(vcpu))

• return 1;
  

• if (hv_result_success(result) && is_guest_mode(vcpu) && kvm_hv_is_tlb_flush_hcall(vcpu) && kvm_read_guest(vcpu->kvm, to_hv_vcpu(vcpu)->nested.pa_page_gpa,

@@ -2919,6 +2938,9 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,

void kvm_hv_vcpu_suspend_tlb_flush(struct kvm_vcpu *vcpu, int vcpu_id) {

• RCU_LOCKDEP_WARN(!srcu_read_lock_held(&vcpu->kvm->srcu),

• 	 "Suspicious Hyper-V TLB flush inhibit usage\n");
  

• /* waiting_on's store should happen before suspended's */ WRITE_ONCE(vcpu->arch.hyperv->waiting_on, vcpu_id); WRITE_ONCE(vcpu->arch.hyperv->suspended, true);

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 18d0a300e79a..1f925e32a927 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4642,6 +4642,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) case KVM_CAP_HYPERV_CPUID: case KVM_CAP_HYPERV_ENFORCE_CPUID: case KVM_CAP_SYS_HYPERV_CPUID:

• case KVM_CAP_HYPERV_TLB_FLUSH_INHIBIT:

#endif case KVM_CAP_PCI_SEGMENT: case KVM_CAP_DEBUGREGS: @@ -5853,6 +5854,31 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu, } }

+static int kvm_vcpu_ioctl_set_tlb_flush_inhibit(struct kvm_vcpu *vcpu,

• 				struct kvm_hyperv_tlb_flush_inhibit *set)
  

+{

• if (set->inhibit == READ_ONCE(vcpu->arch.hyperv->tlb_flush_inhibit))

• return 0;
  

• WRITE_ONCE(vcpu->arch.hyperv->tlb_flush_inhibit, set->inhibit);

As you say before, vCPU ioctls are serialized and noone else sets tlb_flush_inhibit, do I understand correctly that READ_ONCE()/WRITE_ONCE() are redundant here?

As mentioned before, since tlb_flush_inhibit is shared it needs these calls.

Nikolas