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SubjectRe: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED
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On 4/17/24 04:02, Nikolay Borisov wrote:
>
> On 17.04.24 г. 9:30 ч., Xin Li (Intel) wrote:
>> Add a FRED-specific INT80 handler fred_int80_emulation():
>>
>> 1) As INT instructions and hardware interrupts are separate event
>>     types, FRED does not preclude the use of vector 0x80 for external
>>     interrupts. As a result the FRED setup code does *NOT* reserve
>>     vector 0x80 and calling int80_is_external() is not merely
>>     suboptimal but actively incorrect: it could cause a system call
>>     to be incorrectly ignored.
>>
>> 2) fred_int80_emulation(), only called for handling vector 0x80 of
>>     event type EVENT_TYPE_SWINT, will NEVER be called to handle any
>>     external interrupt (event type EVENT_TYPE_EXTINT).
>>
>> 3) The FRED kernel entry handler does *NOT* dispatch INT instructions,
>>     which is of event type EVENT_TYPE_SWINT, so compared with
>>     do_int80_emulation(), there is no need to do any user mode check.
>>
>> 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely
>> >     overkill for new x86 CPU implementations that support FRED.
>
> Well, that's a bit of an overstatement/speculation, because
> clear_branch_history will only be effective if the machine is
> susceptible to the given bug and there isn't a better options (i.e using
> a hardware bit controlling the respective aspect of the CPU).
>>

It would seem like a huge stretch to expect that a FRED-capable CPU
would not have such a facility. This is a matter of establishing a
baseline for FRED-capable hardware.

It would make more sense to me to add it if we turn out to need it; note
that FRED code is currently only enabled on demand, in order to defend
against bit rot until we have physical hardware.

Now, if this is still desired, it *probably* belongs better in either
fred_intx()/fred_other() or asm_fred_entrypoint_user, depending on if
this ought to be done for all entries from userspace or only system calls.

-hpa

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