Re: [PATCH v2] KVM: x86: avoid large stack allocations in em_fxrstor

[kbuild test robot]

Hi Nick,

[auto build test WARNING on kvm/linux-next]
[also build test WARNING on v4.12-rc3 next-20170529]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Nick-Desaulniers/KVM-x86-avoid-large-stack-allocations-in-em_fxrstor/20170530-040058
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git linux-next
config: x86_64-allmodconfig (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

Note: it may well be a FALSE warning. FWIW you are at least aware of it now.
http://gcc.gnu.org/wiki/Better_Uninitialized_Warnings

All warnings (new ones prefixed by >>):

   arch/x86/kvm/emulate.c: In function 'em_fxrstor':
>> arch/x86/kvm/emulate.c:4015:5: warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]
     rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
     ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

vim +/size +4015 arch/x86/kvm/emulate.c

  3999	
  4000		if (ctxt->mode < X86EMUL_MODE_PROT64) {
  4001			rc = asm_safe("fxsave %[fx]", , [fx] "+m"(fx_state));
  4002			if (rc != X86EMUL_CONTINUE)
  4003				return rc;
  4004			/*
  4005			 * Hardware doesn't save and restore XMM 0-7 without
  4006			 * CR4.OSFXSR, but does save and restore MXCSR.
  4007			 */
  4008			if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_OSFXSR)
  4009				size = offsetof(struct fxregs_state, xmm_space[8]);
  4010			else
  4011				size = offsetof(struct fxregs_state, xmm_space[0]);
  4012		} else if (ctxt->mode == X86EMUL_MODE_PROT64)
  4013			size = offsetof(struct fxregs_state, xmm_space[16]);
  4014	
> 4015		rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);
  4016		if (rc != X86EMUL_CONTINUE)
  4017			return rc;
  4018	
  4019		if (fx_state.mxcsr >> 16)
  4020			return emulate_gp(ctxt, 0);
  4021	
  4022		if (rc == X86EMUL_CONTINUE)
  4023			rc = asm_safe("fxrstor %[fx]", : [fx] "m"(fx_state));

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