linux-kernel - Re: [PATCH v1 1/1] x86/fred: Fix int80 emulation for FRED

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Date: Tue, 16 Apr 2024 11:05:09 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Borislav Petkov <bp@...en8.de>, "Xin Li (Intel)" <xin@...or.com>
CC: linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org
Subject: Re: [PATCH v1 1/1] x86/fred: Fix int80 emulation for FRED

On April 16, 2024 3:11:47 AM PDT, Borislav Petkov <bp@...en8.de> wrote:
>On Fri, Apr 12, 2024 at 04:40:58PM -0700, Xin Li (Intel) wrote:
>> Commit 55617fb991df
>
>Use the full commit abbreviation when mentioning commits:
>
>"Commit
>
>  55617fb991df ("x86/entry: Do not allow external 0x80 interrupts")
>
>..."
>
>> added a bunch of tests to the int $0x80 path,
>
>Added a bunch of tests?
>
>What does that even mean?
>
>> however they are unnecessary and event wrong in fact under FRED.
>
>Are the bunch of tests wrong or is do_int80_emulation() simply the wrong
>handler to use on a FRED?
>
>> First FRED distinguishes external interrupts from software interrupts,
>> thus int80_emulation() should NEVER be called for handling an external
>> interrupt, and then int80_is_external() should be skipped under FRED.
>> 
>> Second, the FRED kernel entry handler NEVER dispatches INTx, which is
>> of event type EVENT_TYPE_SWINT, so the user mode checking in
>> do_int80_emulation() is redundant, and should be skipped.
>> 
>> It might be even better to strip down do_int80_emulation() to a lean
>> fred_int80_emulation(), not to mention int80_emulation() does a
>> CLEAR_BRANCH_HISTORY.
>
>Yah, how about you do a FRED-specific INT80 handler instead of
>sprinkling moar tests around? fred_intx() looks like the right place to
>stuff it in...
>

The question was if you wanted a quick fix for x86/urgent. It's pretty obvious that a FRED fork of the int80 code is called for.