Re: [RFC PATCH] KVM: x86: Dynamically allocate bitmap to fix -Wframe-larger-than error

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 13, 2025, Abinash wrote:
> I am building the kernel WITH LLVM.
> KASAN is not enabled.
> CONFIG_FRAME_WARN=1024 (default) . I used defconfig -> my system config ->
> oldconfig to obtain the .config  .
> EXPERT=y So KVM_WERROR=y
> I think this warning is due to FRAME_WARN=1024 .

Well drat, it really is just KVM_MAX_NR_VCPUS=4096.  Luckily, it's quite easy to
fix (we already did this dance for the sparse_banks):

Side topic, please let me know if I got your name right for the Reported-by.

--
From: Sean Christopherson <seanjc@google.com>
Date: Fri, 13 Jun 2025 12:39:22 -0700
Subject: [PATCH] KVM: x86/hyper-v: Use preallocated per-vCPU buffer for
 de-sparsified vCPU masks

Use a preallocated per-vCPU bitmap for tracking the unpacked set of vCPUs
being targeted for Hyper-V's paravirt TLB flushing.  If KVM_MAX_NR_VCPUS
is set to 4096 (which is allowed even for MAXSMP=n builds), putting the
vCPU mask on-stack pushes kvm_hv_flush_tlb() past the default FRAME_WARN
limit.

  arch/x86/kvm/hyperv.c:2001:12: error: stack frame size (1288) exceeds limit (1024)
                                 in 'kvm_hv_flush_tlb' [-Werror,-Wframe-larger-than]
  2001 | static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
       |            ^
  1 error generated.

Note, sparse_banks was given the same treatment by commit 7d5e88d301f8
("KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv'
instead of on-stack 'sparse_banks'"), for the exact same reason.

Reported-by: Abinash Lalotra <abinashsinghlalotra@gmail.com>
Closes: https://lore.kernel.org/all/20250613111023.786265-1-abinashsinghlalotra@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_host.h | 7 ++++++-
 arch/x86/kvm/hyperv.c           | 2 +-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 330cdcbed1a6..12edf36d4ed7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -699,8 +699,13 @@ struct kvm_vcpu_hv {
 
 	struct kvm_vcpu_hv_tlb_flush_fifo tlb_flush_fifo[HV_NR_TLB_FLUSH_FIFOS];
 
-	/* Preallocated buffer for handling hypercalls passing sparse vCPU set */
+	/*
+	 * Preallocated buffers for handling hypercalls that pass sparse vCPU
+	 * sets (for high vCPU counts, they're too large to comfortably fit on
+	 * the stack).
+	 */
 	u64 sparse_banks[HV_MAX_SPARSE_VCPU_BANKS];
+	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 
 	struct hv_vp_assist_page vp_assist_page;
 
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 24f0318c50d7..75221a11e15e 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -2001,11 +2001,11 @@ int kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
 static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
 {
 	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
+	unsigned long *vcpu_mask = hv_vcpu->vcpu_mask;
 	u64 *sparse_banks = hv_vcpu->sparse_banks;
 	struct kvm *kvm = vcpu->kvm;
 	struct hv_tlb_flush_ex flush_ex;
 	struct hv_tlb_flush flush;
-	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
 	struct kvm_vcpu_hv_tlb_flush_fifo *tlb_flush_fifo;
 	/*
 	 * Normally, there can be no more than 'KVM_HV_TLB_FLUSH_FIFO_SIZE'

base-commit: 61374cc145f4a56377eaf87c7409a97ec7a34041
--