linux-kernel - Re: [PATCH] KVM/VMX: Do not declare vmread

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Date:   Thu, 1 Sep 2022 15:39:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM/VMX: Do not declare vmread_error asmlinkage

On Wed, Aug 17, 2022, Uros Bizjak wrote:
> There is no need to declare vmread_error asmlinkage, its arguments
> can be passed via registers for both, 32-bit and 64-bit targets.
> Function argument registers are considered call-clobbered registers,
> they are saved in the trampoline just before the function call and
> restored afterwards.
> 
> Note that asmlinkage and __attribute__((regparm(0))) have no effect
> on 64-bit targets. The trampoline is called from the assembler glue
> code that implements its own stack-passing function calling convention,
> so the attribute on the trampoline declaration does not change anything
> for 64-bit as well as 32-bit targets. We can declare it asmlinkage for
> documentation purposes.

...

> diff --git a/arch/x86/kvm/vmx/vmx_ops.h b/arch/x86/kvm/vmx/vmx_ops.h
> index 5cfc49ddb1b4..550a89394d9f 100644
> --- a/arch/x86/kvm/vmx/vmx_ops.h
> +++ b/arch/x86/kvm/vmx/vmx_ops.h
> @@ -10,9 +10,9 @@
>  #include "vmcs.h"
>  #include "../x86.h"
>  
> -asmlinkage void vmread_error(unsigned long field, bool fault);
> -__attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,
> -							 bool fault);
> +void vmread_error(unsigned long field, bool fault);
> +asmlinkage void vmread_error_trampoline(unsigned long field,
> +					bool fault);
>  void vmwrite_error(unsigned long field, unsigned long value);
>  void vmclear_error(struct vmcs *vmcs, u64 phys_addr);
>  void vmptrld_error(struct vmcs *vmcs, u64 phys_addr);

If it's ok with you, I'll split this into two patches.  One to drop asmlinkage
from vmread_error(), and one to convert the open coded regparm to asmlinkage.