Re: [PATCH v2 04/22] x86/kvm: Don't call kvm_spurious_fault() from .fixup

[Sean Christopherson]

On Thu, Jul 18, 2019 at 10:22:50AM +0200, Paolo Bonzini wrote:
> On 18/07/19 03:36, Josh Poimboeuf wrote:
> > After making a change to improve objtool's sibling call detection, it
> > started showing the following warning:
> > 
> >   arch/x86/kvm/vmx/nested.o: warning: objtool: .fixup+0x15: sibling call from callable instruction with modified stack frame
> > 
> > The problem is the ____kvm_handle_fault_on_reboot() macro.  It does a
> > fake call by pushing a fake RIP and doing a jump.  That tricks the
> > unwinder into printing the function which triggered the exception,
> > rather than the .fixup code.
> > 
> > Instead of the hack to make it look like the original function made the
> > call, just change the macro so that the original function actually does
> > make the call.  This allows removal of the hack, and also makes objtool
> > happy.
> > 
> > I triggered a vmx instruction exception and verified that the stack
> > trace is still sane:
> > 
> >   kernel BUG at arch/x86/kvm/x86.c:358!
> >   invalid opcode: 0000 [#1] SMP PTI
> >   CPU: 28 PID: 4096 Comm: qemu-kvm Not tainted 5.2.0+ #16
> >   Hardware name: Lenovo THINKSYSTEM SD530 -[7X2106Z000]-/-[7X2106Z000]-, BIOS -[TEE113Z-1.00]- 07/17/2017
> >   RIP: 0010:kvm_spurious_fault+0x5/0x10
> >   Code: 00 00 00 00 00 8b 44 24 10 89 d2 45 89 c9 48 89 44 24 10 8b 44 24 08 48 89 44 24 08 e9 d4 40 22 00 0f 1f 40 00 0f 1f 44 00 00 <0f> 0b 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 55 49 89 fd 41
> >   RSP: 0018:ffffbf91c683bd00 EFLAGS: 00010246
> >   RAX: 000061f040000000 RBX: ffff9e159c77bba0 RCX: ffff9e15a5c87000
> >   RDX: 0000000665c87000 RSI: ffff9e15a5c87000 RDI: ffff9e159c77bba0
> >   RBP: 0000000000000000 R08: 0000000000000000 R09: ffff9e15a5c87000
> >   R10: 0000000000000000 R11: fffff8f2d99721c0 R12: ffff9e159c77bba0
> >   R13: ffffbf91c671d960 R14: ffff9e159c778000 R15: 0000000000000000
> >   FS:  00007fa341cbe700(0000) GS:ffff9e15b7400000(0000) knlGS:0000000000000000
> >   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >   CR2: 00007fdd38356804 CR3: 00000006759de003 CR4: 00000000007606e0
> >   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >   PKRU: 55555554
> >   Call Trace:
> >    loaded_vmcs_init+0x4f/0xe0
> >    alloc_loaded_vmcs+0x38/0xd0
> >    vmx_create_vcpu+0xf7/0x600
> >    kvm_vm_ioctl+0x5e9/0x980
> >    ? __switch_to_asm+0x40/0x70
> >    ? __switch_to_asm+0x34/0x70
> >    ? __switch_to_asm+0x40/0x70
> >    ? __switch_to_asm+0x34/0x70
> >    ? free_one_page+0x13f/0x4e0
> >    do_vfs_ioctl+0xa4/0x630
> >    ksys_ioctl+0x60/0x90
> >    __x64_sys_ioctl+0x16/0x20
> >    do_syscall_64+0x55/0x1c0
> >    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >   RIP: 0033:0x7fa349b1ee5b
> > 
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > ---
> > v2: Fix ____kvm_handle_fault_on_reboot() comment [Paolo]
> > 
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 34 ++++++++++++++++++---------------
> >  1 file changed, 19 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 0cc5b611a113..8282b8d41209 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1496,25 +1496,29 @@ enum {
> >  #define kvm_arch_vcpu_memslots_id(vcpu) ((vcpu)->arch.hflags & HF_SMM_MASK ? 1 : 0)
> >  #define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, (role).smm)
> >  
> > +asmlinkage void __noreturn kvm_spurious_fault(void);

With __noreturn added, can the entry in __dead_end_function() in
tools/objtool/check.c be removed?

> > +
> >  /*
> >   * Hardware virtualization extension instructions may fault if a
> >   * reboot turns off virtualization while processes are running.
> > - * Trap the fault and ignore the instruction if that happens.
> > + * Usually after catching the fault we just panic; during reboot
> > + * instead the instruction is ignored.
> >   */
> > -asmlinkage void kvm_spurious_fault(void);
> > -
> > -#define ____kvm_handle_fault_on_reboot(insn, cleanup_insn)	\
> > -	"666: " insn "\n\t" \
> > -	"668: \n\t"                           \
> > -	".pushsection .fixup, \"ax\" \n" \
> > -	"667: \n\t" \
> > -	cleanup_insn "\n\t"		      \
> > -	"cmpb $0, kvm_rebooting \n\t"	      \
> > -	"jne 668b \n\t"      		      \
> > -	__ASM_SIZE(push) " $666b \n\t"	      \
> > -	"jmp kvm_spurious_fault \n\t"	      \
> > -	".popsection \n\t" \
> > -	_ASM_EXTABLE(666b, 667b)
> > +#define ____kvm_handle_fault_on_reboot(insn, cleanup_insn)		\
> > +	"666: \n\t"							\
> > +	insn "\n\t"							\
> > +	"jmp	668f \n\t"						\
> > +	"667: \n\t"							\
> > +	"call	kvm_spurious_fault \n\t"				\
> > +	"668: \n\t"							\
> > +	".pushsection .fixup, \"ax\" \n\t"				\
> > +	"700: \n\t"							\
> > +	cleanup_insn "\n\t"						\
> > +	"cmpb	$0, kvm_rebooting\n\t"					\
> > +	"je	667b \n\t"						\
> > +	"jmp	668b \n\t"						\
> > +	".popsection \n\t"						\
> > +	_ASM_EXTABLE(666b, 700b)
> >  
> >  #define __kvm_handle_fault_on_reboot(insn)		\
> >  	____kvm_handle_fault_on_reboot(insn, "")
> > 
> 
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> 
> This has a side effect of adding a jump in a generally hot path, but
> let's hope that the speculation gods for once help us.

Any reason not to take the same approach as vmx_vmenter() and ud2 directly
from fixup?  I've never found kvm_spurious_fault() to be all that helpful,
IMO it's a win win. :-)