Re: [PATCH v2] KVM: VMX: Improve handle_external_interrupt_irqoff inline assembly

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Sun, Apr 26, 2020 at 01:52:55PM +0200, Uros Bizjak wrote:
> Improve handle_external_interrupt_irqoff inline assembly in several ways:
> - use "n" operand constraint instead of "i" and remove

What's the motivation for using 'n'?  The 'i' variant is much more common,
i.e. less likely to trip up readers.

  $ git grep -E "\"i\"\s*\(" | wc -l
  768
  $ git grep -E "\"n\"\s*\(" | wc -l
  11

>   unneeded %c operand modifiers and "$" prefixes
> - use %rsp instead of _ASM_SP, since we are in CONFIG_X86_64 part
> - use $-16 immediate to align %rsp

Heh, this one depends on the reader, I find 0xfffffffffffffff0 to be much
more intuitive, though admittedly also far easier to screw up.

> - remove unneeded use of __ASM_SIZE macro
> - define "ss" named operand only for X86_64
> 
> The patch introduces no functional changes.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c2c6335a998c..7471f1b948b3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6283,13 +6283,13 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
>  
>  	asm volatile(
>  #ifdef CONFIG_X86_64
> -		"mov %%" _ASM_SP ", %[sp]\n\t"
> -		"and $0xfffffffffffffff0, %%" _ASM_SP "\n\t"
> -		"push $%c[ss]\n\t"
> +		"mov %%rsp, %[sp]\n\t"
> +		"and $-16, %%rsp\n\t"
> +		"push %[ss]\n\t"
>  		"push %[sp]\n\t"
>  #endif
>  		"pushf\n\t"
> -		__ASM_SIZE(push) " $%c[cs]\n\t"
> +		"push %[cs]\n\t"
>  		CALL_NOSPEC
>  		:
>  #ifdef CONFIG_X86_64
> @@ -6298,8 +6298,10 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
>  		ASM_CALL_CONSTRAINT
>  		:
>  		[thunk_target]"r"(entry),
> -		[ss]"i"(__KERNEL_DS),
> -		[cs]"i"(__KERNEL_CS)
> +#ifdef CONFIG_X86_64
> +		[ss]"n"(__KERNEL_DS),
> +#endif
> +		[cs]"n"(__KERNEL_CS)
>  	);
>  
>  	kvm_after_interrupt(vcpu);
> -- 
> 2.25.3
>