linux-kernel - Re: vmlinux.o: warning: objtool: __svm_sev_es_vcpu_run()+0x54: BP used as a scratch register

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Message-ID: <Yc0IyjWqEjqx56To@google.com>
Date:   Thu, 30 Dec 2021 01:18:02 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     kernel test robot <lkp@...el.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: vmlinux.o: warning: objtool: __svm_sev_es_vcpu_run()+0x54: BP
 used as a scratch register

On Wed, Nov 17, 2021, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   8ab774587903771821b59471cc723bba6d893942
> commit: 16809ecdc1e8ab7278f1d60021ac809edd17d060 KVM: SVM: Provide an updated VMRUN invocation for SEV-ES guests
> date:   11 months ago
> config: x86_64-buildonly-randconfig-r005-20211117 (attached as .config)
> compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
> reproduce (this is a W=1 build):
>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=16809ecdc1e8ab7278f1d60021ac809edd17d060
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout 16809ecdc1e8ab7278f1d60021ac809edd17d060
>         # save the attached .config to linux build tree
>         mkdir build_dir
>         make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
> 
> All warnings (new ones prefixed by >>):
> 
>    vmlinux.o: warning: objtool: __do_fast_syscall_32()+0x3d: call to syscall_enter_from_user_mode_work() leaves .noinstr.text section
>    vmlinux.o: warning: objtool: __svm_vcpu_run()+0x10e: BP used as a scratch register
> >> vmlinux.o: warning: objtool: __svm_sev_es_vcpu_run()+0x54: BP used as a scratch register

__svm_vcpu_run() and __svm_sev_es_vcpu_run() PUSH/POP RBP but don't create a stack
frame, which triggers the objtool warning because objtool things the code is using
BP as a scratch register (obviously).  This shows up when CONFIG_FRAME_POINTER=y
and KVM AMD is built into the kernel (apparently 'objtool check' isn't run on modules?).

The easy fix to is add "mov %rsp, %rbp" to create a stack frame.  Kinda pointless
since neither function makes true calls to other functions, but the extra MOV is
harmless and it'd be nice to get rid of the objtool warning.