Re: [PATCH 1/2] KVM: VMX: Move IRQ invocation to assembly subroutine

[Josh Poimboeuf <jpoimboe@redhat.com>]

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 <jpoimboe@redhat.com>
> Cc: Uros Bizjak <ubizjak@gmail.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@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