Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts

Masami Hiramatsu
2020-04-10T11:25:48Z

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>