Re: [stable:linux-5.15.y 55/9999] arch/x86/kvm/hyperv.c:2185:5: error: stack frame size (1036) exceeds limit (1024) in 'kvm_hv_hypercall'

[Vitaly Kuznetsov <vkuznets@redhat.com>]

kernel test robot <lkp@intel.com> writes:

> Hi Vitaly,
>
> First bad commit (maybe != root cause):
>
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git linux-5.15.y
> head:   d54cfc420586425d418a53871290cc4a59d33501
> commit: cb188e07105f2216f5efbefac95df4b6ce266906 [55/9999] KVM: x86: hyper-v: HVCALL_SEND_IPI_EX is an XMM fast hypercall
> config: i386-buildonly-randconfig-r006-20230708 (https://download.01.org/0day-ci/archive/20230708/202307080326.zDp7E3o0-lkp@intel.com/config)
> compiler: clang version 15.0.7 (https://github.com/llvm/llvm-project.git 8dfdcc7b7bf66834a761bd8de445840ef68e4d1a)
> reproduce: (https://download.01.org/0day-ci/archive/20230708/202307080326.zDp7E3o0-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202307080326.zDp7E3o0-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
>>> arch/x86/kvm/hyperv.c:2185:5: error: stack frame size (1036) exceeds limit (1024) in 'kvm_hv_hypercall' [-Werror,-Wframe-larger-than]
>    int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>        ^
>    1 error generated.

(sorry for delayed reply)

This used to be a warning (without CONFIG_KVM_WERROR I guess?) :-) E.g.

https://lore.kernel.org/kvm/87zgg6sza8.fsf@redhat.com/#t

where Nathan explained LLVM's behavior:

https://lore.kernel.org/kvm/Yvp87jlVWg0e376v@dev-arch.thelio-3990X/

This was 'fixed' upstream with

commit 7d5e88d301f84a7b64602dbe3640f288223095ea
Author: Vitaly Kuznetsov <vkuznets@redhat.com>
Date:   Tue Nov 1 15:53:56 2022 +0100

    KVM: x86: hyper-v: Use preallocated buffer in 'struct kvm_vcpu_hv' instead of on-stack 'sparse_banks'
  
and personally, I'm not against backporting it to 5.15.y but I seriously
doubt it is worth the hassle (i386 KVM + llvm + CONFIG_KVM_WERROR is
likely an impossible combo).

Also, there seems to be another build problem with CONFIG_KVM_WERROR I
met with clan-16 and the same config:

../arch/x86/kvm/x86.c:2315:19: error: unused function 'gtod_is_based_on_tsc' [-Werror,-Wunused-function]
static inline int gtod_is_based_on_tsc(int mode)

TL;DR: Let's ignore this for 5.15, not worth fixing IMO. Cc: kvm@ to
check if anyone thinks differently.

>
>
> vim +/kvm_hv_hypercall +2185 arch/x86/kvm/hyperv.c
>
> 4ad81a91119df7 Vitaly Kuznetsov         2021-05-21  2184  
> e83d58874ba1de Andrey Smetanin          2015-07-03 @2185  int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
> e83d58874ba1de Andrey Smetanin          2015-07-03  2186  {
> 4e62aa96d6e55c Vitaly Kuznetsov         2021-07-30  2187  	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> bd38b32053eb1c Siddharth Chandrasekaran 2021-05-26  2188  	struct kvm_hv_hcall hc;
> bd38b32053eb1c Siddharth Chandrasekaran 2021-05-26  2189  	u64 ret = HV_STATUS_SUCCESS;
> e83d58874ba1de Andrey Smetanin          2015-07-03  2190  
> e83d58874ba1de Andrey Smetanin          2015-07-03  2191  	/*
> e83d58874ba1de Andrey Smetanin          2015-07-03  2192  	 * hypercall generates UD from non zero cpl and real mode
> e83d58874ba1de Andrey Smetanin          2015-07-03  2193  	 * per HYPER-V spec
> e83d58874ba1de Andrey Smetanin          2015-07-03  2194  	 */
> b3646477d458fb Jason Baron              2021-01-14  2195  	if (static_call(kvm_x86_get_cpl)(vcpu) != 0 || !is_protmode(vcpu)) {
> e83d58874ba1de Andrey Smetanin          2015-07-03  2196  		kvm_queue_exception(vcpu, UD_VECTOR);
> 0d9c055eaaf41b Andrey Smetanin          2016-02-11  2197  		return 1;
> e83d58874ba1de Andrey Smetanin          2015-07-03  2198  	}
> e83d58874ba1de Andrey Smetanin          2015-07-03  2199  
>
> :::::: The code at line 2185 was first introduced by commit
> :::::: e83d58874ba1de74c13d3c6b05f95a023c860d25 kvm/x86: move Hyper-V MSR's/hypercall code into hyperv.c file
>
> :::::: TO: Andrey Smetanin <asmetanin@virtuozzo.com>
> :::::: CC: Paolo Bonzini <pbonzini@redhat.com>

-- 
Vitaly