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

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

[Uros Bizjak]

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.

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

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

[Sean Christopherson]

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
> 

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

[Uros Bizjak]

On Mon, May 4, 2020 at 5:25 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> 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?

I think this would be a good idea. So, in this patch the first point should read

"- remove unneeded %c operand modifiers and "$" prefixes"

The add-on patch will then explain that PUSH can only handle signed
32bit immediates and change "i" to "re".

Is this what you had in mind?

Thanks,
Uros.

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

[Sean Christopherson]

On Mon, May 04, 2020 at 05:32:19PM +0200, Uros Bizjak wrote:
> On Mon, May 4, 2020 at 5:25 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > 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 $-16 immediate to align %rsp
> > > - remove unneeded use of __ASM_SIZE macro
> > > - define "ss" named operand only for X86_64
> > >
> > > The patch introduces no functional changes.
> >
> > Hmm, for handcoded assembly I would argue that the switch from "i" to "re"
> > is a functional change of sorts.  The switch also needs explicit
> > justification to explain why it's correct/desirable.  Maybe make it a
> > separate patch?
> 
> I think this would be a good idea. So, in this patch the first point should read
> 
> "- remove unneeded %c operand modifiers and "$" prefixes"
> 
> The add-on patch will then explain that PUSH can only handle signed
> 32bit immediates and change "i" to "re".
> 
> Is this what you had in mind?

Yep, exactly.