Re: [PATCH] KVM: x86: (64-bit x86_64 machine) : fix -frame-larger-than warnings/errors.

[Sean Christopherson <seanjc@google.com>]

On Sun, Oct 24, 2021, Ajay Garg wrote:
> Hi Sean.
> 
> Today I enabled a debug-build, and the compilation broke again.
> 
> 1.
> Please find attached the .config  file.

Aha!  The problem is CONFIG_KASAN_STACK=y, which is selected (and can't be
unselected) by CONFIG_KASAN=y when compiling with gcc (clang/LLVM is a stack hog
in some cases so it's opt-in for clang).  KASAN_STACK adds a redzone around every
stack variable, which pushes the Hyper-V functions over the limit.

> Please let know if/when I should float a v2 patch.

I still don't love hoisting sparse_banks up a level, that info really shouldn't
bleed into the caller.  My alternative solution is to push vp_bitmap down into
sparse_set_to_vcpu_mask().  That doesn't "free" up as much stack, but it's enough
to get under the 1024 byte default.  It's also nice in that it hides the VP
index mismatch logic in sparse_set_to_vcpu_mask().

I'll post a proper patch tomorrow (completely untested):

---
 arch/x86/kvm/hyperv.c | 55 ++++++++++++++++++++++---------------------
 1 file changed, 28 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 4f15c0165c05..80018cfab5c7 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1710,31 +1710,36 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
 		return kvm_hv_get_msr(vcpu, msr, pdata, host);
 }

-static __always_inline unsigned long *sparse_set_to_vcpu_mask(
-	struct kvm *kvm, u64 *sparse_banks, u64 valid_bank_mask,
-	u64 *vp_bitmap, unsigned long *vcpu_bitmap)
+static void sparse_set_to_vcpu_mask(struct kvm *kvm, u64 *sparse_banks,
+				    u64 valid_bank_mask, unsigned long *vcpu_mask)
 {
 	struct kvm_hv *hv = to_kvm_hv(kvm);
+	bool has_mismatch = atomic_read(&hv->num_mismatched_vp_indexes);
+	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
 	struct kvm_vcpu *vcpu;
 	int i, bank, sbank = 0;
+	u64 *bitmap;

-	memset(vp_bitmap, 0,
-	       KVM_HV_MAX_SPARSE_VCPU_SET_BITS * sizeof(*vp_bitmap));
+	BUILD_BUG_ON(sizeof(vp_bitmap) >
+		     sizeof(*vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS));
+
+	/* If vp_index == vcpu_idx for all vCPUs, fill vcpu_mask directly. */
+	if (likely(!has_mismatch))
+		bitmap = (u64 *)vcpu_mask;
+
+	memset(bitmap, 0, sizeof(vp_bitmap));
 	for_each_set_bit(bank, (unsigned long *)&valid_bank_mask,
 			 KVM_HV_MAX_SPARSE_VCPU_SET_BITS)
-		vp_bitmap[bank] = sparse_banks[sbank++];
+		bitmap[bank] = sparse_banks[sbank++];

-	if (likely(!atomic_read(&hv->num_mismatched_vp_indexes))) {
-		/* for all vcpus vp_index == vcpu_idx */
-		return (unsigned long *)vp_bitmap;
-	}
+	if (likely(!has_mismatch))
+		return;

-	bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS);
+	bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (test_bit(kvm_hv_get_vpindex(vcpu), (unsigned long *)vp_bitmap))
-			__set_bit(i, vcpu_bitmap);
+			__set_bit(i, vcpu_mask);
 	}
-	return vcpu_bitmap;
 }

 struct kvm_hv_hcall {
@@ -1756,9 +1761,7 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
 	struct kvm *kvm = vcpu->kvm;
 	struct hv_tlb_flush_ex flush_ex;
 	struct hv_tlb_flush flush;
-	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
-	DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);
-	unsigned long *vcpu_mask;
+	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 	u64 valid_bank_mask;
 	u64 sparse_banks[64];
 	int sparse_banks_len;
@@ -1842,11 +1845,9 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
 	if (all_cpus) {
 		kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH_GUEST);
 	} else {
-		vcpu_mask = sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask,
-						    vp_bitmap, vcpu_bitmap);
+		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask);

-		kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST,
-					    vcpu_mask);
+		kvm_make_vcpus_request_mask(kvm, KVM_REQ_TLB_FLUSH_GUEST, vcpu_mask);
 	}

 ret_success:
@@ -1879,9 +1880,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
 	struct kvm *kvm = vcpu->kvm;
 	struct hv_send_ipi_ex send_ipi_ex;
 	struct hv_send_ipi send_ipi;
-	u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
-	DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);
-	unsigned long *vcpu_mask;
+	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 	unsigned long valid_bank_mask;
 	u64 sparse_banks[64];
 	int sparse_banks_len;
@@ -1937,11 +1936,13 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
 	if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR))
 		return HV_STATUS_INVALID_HYPERCALL_INPUT;

-	vcpu_mask = all_cpus ? NULL :
-		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask,
-					vp_bitmap, vcpu_bitmap);
+	if (all_cpus) {
+		kvm_send_ipi_to_many(kvm, vector, NULL);
+	} else {
+		sparse_set_to_vcpu_mask(kvm, sparse_banks, valid_bank_mask, vcpu_mask);

-	kvm_send_ipi_to_many(kvm, vector, vcpu_mask);
+		kvm_send_ipi_to_many(kvm, vector, vcpu_mask);
+	}

 ret_success:
 	return HV_STATUS_SUCCESS;
--
2.33.0.1079.g6e70778dc9-goog