Re: [PATCH v4] KVM: VMX: Improve handle_external_interrupt_irqoff inline assembly

Sean Christopherson
2020-05-04T15:25:22Z

On Mon, May 04, 2020 at 01:05:45AM +0200, Uros Bizjak wrote:
> Improve handle_external_interrupt_irqoff inline assembly in several ways:
> - use "re" operand constraint instead of "i" and remove
> 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
> - remove unneeded use of __ASM_SIZE macro
> - define "ss" named operand only for X86_64
>
> The patch introduces no functional changes.

Hmm, for handcoded assembly I would argue that the switch from "i" to "re"
is a functional change of sorts. The switch also needs explicit
justification to explain why it's correct/desirable. Maybe make it a
separate patch?

> 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..56c742effb30 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]"re"(__KERNEL_DS),
> +#endif
> + [cs]"re"(__KERNEL_CS)
> );
>
> kvm_after_interrupt(vcpu);
> --
> 2.25.4
>