Re: [linux-stable-rc:linux-5.15.y 5373/8464] arch/x86/kvm/hyperv.c:2185:5: warning: stack frame size (1036) exceeds limit (1024) in 'kvm_hv_hypercall'

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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

> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git linux-5.15.y
> head:   baefa2315cb1371486f6661a628e96fa3336f573
> commit: cb188e07105f2216f5efbefac95df4b6ce266906 [5373/8464] KVM: x86: hyper-v: HVCALL_SEND_IPI_EX is an XMM fast hypercall
> config: i386-allyesconfig (https://download.01.org/0day-ci/archive/20220716/202207161843.WnHPjB0l-lkp@intel.com/config)
> compiler: clang version 15.0.0 (https://github.com/llvm/llvm-project 07022e6cf9b5b3baa642be53d0b3c3f1c403dbfd)
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git/commit/?id=cb188e07105f2216f5efbefac95df4b6ce266906
>         git remote add linux-stable-rc https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git
>         git fetch --no-tags linux-stable-rc linux-5.15.y
>         git checkout cb188e07105f2216f5efbefac95df4b6ce266906
>         # save the config file
>         mkdir build_dir && cp config build_dir/.config
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=i386 SHELL=/bin/bash arch/x86/kvm/
>
> If you fix the issue, kindly add following tag where applicable
> Reported-by: kernel test robot <lkp@intel.com>
>
> All warnings (new ones prefixed by >>):
>
>>> arch/x86/kvm/hyperv.c:2185:5: warning: stack frame size (1036) exceeds limit (1024) in 'kvm_hv_hypercall' [-Wframe-larger-than]
>    int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>        ^
>    1 warning generated.
>
>
> 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;

That's a bit weird: struct kvm_hv_hcall is 144 bytes only so this is
very, very far from 1024. The referred commit (cb188e07105f) also
doesn't add any on-stack allocations to kvm_hv_hypercall() directly,
however, it leaves only once call site for kvm_hv_send_ipi() and the
compiler may have switched to inlining it. Assuming that's the case, I'm
completely clueless about why such potentially dangerous 'optimization'
make any sense.

In any case, there's a pending patch:

https://lore.kernel.org/kvm/20220714134929.1125828-13-vkuznets@redhat.com/

which is supposed to help here.

> 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