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Date: Thu, 18 Jul 2019 06:16:54 -0700
From: Sean Christopherson <sean.j.christopherson@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
Randy Dunlap <rdunlap@...radead.org>,
Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 04/22] x86/kvm: Don't call kvm_spurious_fault() from
.fixup
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@...hat.com>
> > Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > v2: Fix ____kvm_handle_fault_on_reboot() comment [Paolo]
> >
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Krčmář <rkrcmar@...hat.com>
> > ---
> > 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@...hat.com>
>
> 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. :-)
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