On Fri, Sep 17, 2021, Ajay Garg wrote:
> From: ajay <ajaygargnsit@xxxxxxxxx>
>
> Issue :
> =======
>
> In "kvm_hv_flush_tlb" and "kvm_hv_send_ipi" methods, defining
> "u64 sparse_banks[64]" inside the methods (on the stack), causes the
> stack-segment-memory-allocation to go beyond 1024 bytes, thus raising the
> warning/error which breaks the build.
>
> Fix :
> =====
>
> Instead of defining "u64 sparse_banks [64]" inside the methods, we instead
> define this array in the (only) client method "kvm_hv_hypercall", and then
> pass the array (and its size) as additional arguments to the two methods.
> Doing this, we do not exceed the 1024 bytes stack-segment-memory-allocation,
> on any stack-segment of any method.
This is a hack, and it's not guaranteed to work, e.g. if the compiler decided to
inline the helpers, then presumably this problem would rear its head again.
However, I don't think this is a problem any more. gcc-10 and clang-11 are both
comfortably under 1024, even if I force both helpers to be inlined. Neither
function has variables that would scale with NR_CPUS (and I verified high number
of NR_CPUS for giggles). Can you try reproducing the behavior on the latest
kvm/queue? I swear I've seen this in the past, but I couldn't find a commit that
"fixed" any such warning.
If it does repro, can you provide your .config and compiler version? Maybe your
compiler is doing somethign funky?
> Signed-off-by: ajay <ajaygargnsit@xxxxxxxxx>
The SoB needs your full name.
> ---
> arch/x86/kvm/hyperv.c | 34 ++++++++++++++++++++++++----------
> 1 file changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 232a86a6faaf..5340be93daa4 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1750,7 +1750,8 @@ struct kvm_hv_hcall {
> sse128_t xmm[HV_HYPERCALL_MAX_XMM_REGISTERS];
> };
>
> -static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool ex)
> +static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc,
> + bool ex, u64 *sparse_banks, u32 num_sparse_banks)
> {
> int i;
> gpa_t gpa;
> @@ -1762,10 +1763,11 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc, bool
> DECLARE_BITMAP(vcpu_bitmap, KVM_MAX_VCPUS);
> unsigned long *vcpu_mask;
> u64 valid_bank_mask;
> - u64 sparse_banks[64];
> int sparse_banks_len;
> bool all_cpus;
>
> + memset(sparse_banks, 0, sizeof(u64) * num_sparse_banks);
> +
FWIW, the array size needs to be validated, there is other code in this function
that assumes it's at least 64 entries.
