Re: [PATCH 05/12] x86/io-apic: purge usage of logical mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.11.2025 14:18, Andrew Cooper wrote:
> On 20/11/2025 9:06 am, Roger Pau Monne wrote:
>> The IO-APIC RTEs are unconditionally programmed with physical destination
>> mode, and hence the field to set in the RTE is always physical_dest.
>>
>> Remove the mode parameter from SET_DEST() and take the opportunity to
>> convert it into a function, there's no need for it to be a macro.
>>
>> This is a benign fix, because due to the endianness of x86 the start of the
>> physical_dest and logical_dest fields on the RTE overlap.
> 
> RTEs do not have overlapping fields; it's Xen's abstraction of the
> IO-APIC which is buggy.

I wouldn't put it this negatively. In the old days, ...

> For starters, Xen's IO_APIC_route_entry still only has a 4-bit
> physical_dest field which hasn't been true since the Pentium 4 days. 
> This might explain some of the interrupt bugs we see.

... as you mention here, the two fields were distinct (and hence overlapping).
In a number of places we passed "logical" to SET_DEST() as the middle argument,
thus covering for the too narrow field width of physical_dest. Dropping that
parameter and always using physical_dest requires that field to be widened,
though (or else we'll end up chopping off the top 4 bits, as we already do in
disable_IO_APIC() and unlock_ExtINT_logic() - both benign as long as the CPU
used always has APIC ID 0, which will at least typically be the case, I think).

Jan