linux-kernel - Re: [PATCHv2 2/3] x86/tdx: Clarify RIP adjustments in #VE handler

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Message-ID: <af726924-4eb1-aa2c-319f-0a67003ef37f@intel.com>
Date:   Fri, 20 May 2022 10:52:40 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org
Cc:     ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, seanjc@...gle.com,
        thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCHv2 2/3] x86/tdx: Clarify RIP adjustments in #VE handler

On 5/19/22 20:13, Kirill A. Shutemov wrote:
> +	/*
> +	 * If the #VE happened due to instruction execution, GET_VEINFO
> +	 * provides info on the instruction in out.r10.
> +	 *
> +	 * For #VE due to EPT violation, info in out.r10 is not usable and
> +	 * kernel has to decode instruction manually to find out its length.
> +	 */
> +	if (ve->exit_reason != EXIT_REASON_EPT_VIOLATION) {
> +		ve->instr_len   = lower_32_bits(out.r10);
> +		ve->instr_info  = upper_32_bits(out.r10);
> +	} else {
> +		ve->instr_len   = 0;
> +		ve->instr_info  = 0;
> +	}
>  }

This is _better_, but I still don't like it.  It still hides the magic
down in here and if someone screws it up, they'll silently get a loop of
#VE's.  If we stick the logic a helper like:

int ve_instr_len(struct ve_info *ve)
{
	if (WARN_ON_ONCE(ve->exit_reason != EXIT_REASON_EPT_VIOLATION))
		return 0;

	return ve->instr_len;
}

and then folks consume the instruction length with that, we get a splat
right where and when it goes wrong.

BTW, how do we know that all non-EPT_VIOLATION exits reasons are
instruction execution?