linux-kernel - Re: [PATCH 2/4] x86/vmware: Add a header file for hypercall definitions

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Date:   Tue, 20 Aug 2019 15:34:25 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, pv-drivers@...are.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Doug Covelli <dcovelli@...are.com>
Subject: Re: [PATCH 2/4] x86/vmware: Add a header file for hypercall
 definitions

On 8/20/19 1:32 PM, Peter Zijlstra wrote:
> On Sun, Aug 18, 2019 at 04:33:14PM +0200, Thomas Hellström (VMware) wrote:
>
>> +#define VMWARE_HYPERCALL \
>> +	ALTERNATIVE_2(".byte 0xed", \
>> +		      ".byte 0x0f, 0x01, 0xc1", X86_FEATURE_VMW_VMCALL,	\
>> +		      ".byte 0x0f, 0x01, 0xd9", X86_FEATURE_VMW_VMMCALL)
> For sanity, could we either add comments, or macros for those
> instrucions?

Hmm. Here I followed and slightly extended what was done in asm/kvm_para.h.

What confuses me a bit is, if it's clarity we're after, why don't people use

#define VMWARE_HYPERCALL 					\
	ALTERNATIVE_2("inl (%%dx)", 				\
		      "vmcall", X86_FEATURE_VMW_VMCALL,		\	
		      "vmmcall", X86_FEATURE_VMW_VMMCALL)

Seems to build fine here. Is it fear of old assemblers not supporting, for example vmmcall

Thanks,
/Thomas
  

> Something like:
>
> #define INSN_INL	0xed
> #define INSN_VMCALL	0x0f,0x01,0xc1
> #define INSN_VMMCALL	0x0f,0x01,0xd9
>
> #define VMWARE_HYPERCALL \
> 	ALTERNATIVE_2(_ASM_MK_NOP(INSN_INL),
> 		      _ASM_MK_NOP(INSN_VMCALL), X86_FEATURE_VMCALL,
> 		      _ASM_MK_NOP(INSN_VMMCALL), X86_FEATURE_VMMCALL)
>
> With possibly a patch that does 's/_ASM_MK_NOP/_ASM_MK_INSN/' on
> arch/x86/ for further sanity :-)