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Date:   Fri, 10 Apr 2020 20:25:38 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
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Subject: Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts
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On Wed, 8 Apr 2020 11:56:04 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Apr 08, 2020 at 05:09:34PM +0900, Masami Hiramatsu wrote:
> > On Tue, 07 Apr 2020 13:02:39 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > > +static bool insn_is_vmx(struct insn *insn)
> > > +{
> > > +	u8 modrm = insn->modrm.bytes[0];
> > > +	u8 modrm_mod = X86_MODRM_MOD(modrm);
> > > +	u8 modrm_reg = X86_MODRM_REG(modrm);
> > > +
> > > +	u8 prefix = insn->prefixes.bytes[0];
> > 
> > This should be the last prefix,
> > 
> > 	u8 prefix = insn->prefixes.bytes[3];
> > 
> > (The last prefix always copied on the bytes[3])
> 
> And that is 0 on no-prefix, right?

Yes, it should be.

> > > +
> > > +	if (insn->opcode.bytes[0] != 0x0f)
> > > +		return false;
> > > +
> > > +	switch (insn->opcode.bytes[1]) {
> > > +	case 0x01:
> > > +		switch (insn->opcode.bytes[2]) {
> > 
> > Sorry, VMCALL etc. is in Grp7 (0f 01), the 3rd code is embedded
> > in modrm instead of opcode. Thus it should be,
> > 
> > 		switch (insn->modrm.value) {
> 
> Indeed, I was hoping (I really should've checked) that that byte was
> duplicated in opcodes.
> 
> Also, since I already have modrm = insn->modrm.bytes[0], I should
> probably use that anyway.

Yeah, and please use modrm.value instead of bytes[0].
(maybe bytes[0] will be OK since x86 is little-endian)

> > > +		case 0xc1: /* VMCALL */
> > > +		case 0xc2: /* VMLAUNCH */
> > > +		case 0xc3: /* VMRESUME */
> > > +		case 0xc4: /* VMXOFF */
> > 
> > 		case 0xd4:	/* VMFUNC */
> 
> As per Andrew, VMCALL and VMFUNC are SMV, and I really only need VMX in
> this case. Including SMV is probably harmless, but I'm thinking a
> smaller function is better.

I got it.

> 
> > > +			return true;
> > > +
> > > +		default:
> > > +			break;
> > > +		}
> > > +		break;
> > > +
> > > +	case 0x78: /* VMREAD */
> > > +	case 0x79: /* VMWRITE */
> > 
> > 		return !insn_is_evex(insn);
> > 
> > With EVEX prefix, these becomes vcvt* instructions.
> 
> Thanks!


Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>