On Mon, Sep 14, 2020 at 12:56:33PM -0700, Sean Christopherson wrote:
> Move the asm blob that invokes the appropriate IRQ handler after VM-Exit
> into a proper subroutine. Slightly rework the blob so that it plays
> nice with objtool without any additional hints (existing hints aren't
> able to handle returning with a seemingly modified stack size).
>
> Suggested-by: Josh Poimboeuf <jpoim...@redhat.com>
> Cc: Uros Bizjak <ubiz...@gmail.com>
> Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com>
> ---
> arch/x86/kvm/vmx/vmenter.S | 28 ++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/vmx.c | 33 +++------------------------------
> 2 files changed, 31 insertions(+), 30 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 799db084a336..baec1e0fefc5 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -4,6 +4,7 @@
> #include <asm/bitsperlong.h>
> #include <asm/kvm_vcpu_regs.h>
> #include <asm/nospec-branch.h>
> +#include <asm/segment.h>
>
> #define WORD_SIZE (BITS_PER_LONG / 8)
>
> @@ -294,3 +295,30 @@ SYM_FUNC_START(vmread_error_trampoline)
>
> ret
> SYM_FUNC_END(vmread_error_trampoline)
> +
> +SYM_FUNC_START(vmx_do_interrupt_nmi_irqoff)
> + /*
> + * Unconditionally create a stack frame. RSP needs to be aligned for
> + * x86-64, getting the correct RSP on the stack (for x86-64) would take
> + * two instructions anyways, and it helps make objtool happy (see
> below).
> + */
> + push %_ASM_BP
> + mov %rsp, %_ASM_BP
RSP needs to be aligned to what? How would this align the stack, other
than by accident?
> +
> +#ifdef CONFIG_X86_64
> + push $__KERNEL_DS
> + push %_ASM_BP
> +#endif
> + pushf
> + push $__KERNEL_CS
> + CALL_NOSPEC _ASM_ARG1
> +
> + /*
> + * "Restore" RSP from RBP, even though IRET has already unwound RSP to
> + * the correct value. objtool doesn't know the target will IRET and so
> + * thinks the stack is getting walloped (without the explicit restore).
> + */
> + mov %_ASM_BP, %rsp
> + pop %_ASM_BP
> + ret
BTW, there *is* actually an unwind hint for this situation:
UNWIND_HINT_RET_OFFSET.
So you might be able to do something like the following (depending on
what your alignment requirements actually are):
SYM_FUNC_START(vmx_do_interrupt_nmi_irqoff)
#ifdef CONFIG_X86_64
push $__KERNEL_DS
push %_ASM_BP
#endif
pushf
push $__KERNEL_CS
CALL_NOSPEC _ASM_ARG1
/* The call popped the pushes */
UNWIND_HINT_RET_OFFSET sp_offset=32
ret
SYM_FUNC_END(vmx_do_interrupt_nmi_irqoff)
--
Josh
