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

[Xin Li <xin@zytor.com>]

On 4/16/2024 3:11 AM, Borislav Petkov 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")
> 
> .."

oh, I should have done it!

> 
>> added a bunch of tests to the int $0x80 path,
> 
> Added a bunch of tests?
> 

'checks' might be a better term to use?

> 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?

I think the explanations are below, and you saw it a bit later.

BTW, comments are added around the code changes.

> 
>> 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...
> 

I will add fred_int80_emulation(), which duplicates a big part of
do_int80_emulation(), however this seems more readable.

Thanks!
     Xin