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Subject: Re: [PATCH v1 00/19] x86/insn: Add an insn_decode() API
To:     Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Masami Hiramatsu <mhiramat@kernel.org>
Cc:     X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
References: <20201223174233.28638-1-bp@alien8.de>
From:   Tom Lendacky <thomas.lendacky@amd.com>
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Date:   Sun, 27 Dec 2020 09:26:04 -0600
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On 12/23/20 11:42 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
> 
> Hi,
> 
> here's v1 with the requested change to return -ENODATA on short input to
> the decoder. The rest is as in the previous submission.
> 
> Only lightly tested.
> 
> Thx.
> 
> changelog:
> ==========
> 
> That is, provided this is how we want to control what the instruction
> decoder decodes - by supplying the length of the buffer it should look
> at.
> 
> We could also say that probably there should be a way to say "decode
> only the first insn in the buffer and ignore the rest". That is all up
> to the use cases so I'm looking for suggestions here.

That's the way it works today, right?  One instruction, no matter the 
length of the buffer (assuming the length is long enough to include a full 
instruction)?

Because the callers of the decode may rely on parsing only the current 
instruction (like SEV-ES), it should probably default to that (although 
most of the call points are being updated so you could supply a boolean to 
indicate one vs many instructions). The caller doesn't necessarily know 
the length of the instruction, so it may provide a buffer of max 
instruction length.

Also, if you want to parse more than one instruction at a time, wouldn't 
you need to maintain register context within the parsing, I don't think 
that is done today. Or you could chain together some instruction contexts 
to identify each instruction that was parsed?

Thanks,
Tom

>