linux-kernel - Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

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Date:   Fri, 31 Mar 2023 08:30:22 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Wu Zongyong <wuzongyong@...ux.alibaba.com>, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     thomas.lendacky@....com, tony.luck@...el.com,
        kirill.shutemov@...ux.intel.com, wutu.xq2@...ux.alibaba.com
Subject: Re: [RFC PATCH] x86/insn: support decode MOVSXD instruction for MMIO

On 3/28/23 19:59, Wu Zongyong wrote:
> It seems MOVSXD which opcode is 0x63 is not handled, support
> to decode it in insn_decode_mmio().
...
>  	switch (insn->opcode.bytes[0]) {
> +	case 0x63: /* MOVSXD r64, m32 */
> +		*bytes = 4;
> +		type = INSN_MMIO_READ_SIGN_EXTEND;
> +		break;

The kernel does not support _arbitrary_ memory access instructions
messing with MMIO.

Before even considering this, I'd want to see a very concrete
explanation for why _this_ instruction in particular is required.  I'd
also want to make sure this doesn't set us off down a slippery slope
trying to make the MMIO decoder more expansive.