Re: [PATCH] x86/io-apic: fix directed EOI when using AMd-Vi interrupt remapping

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.10.2024 14:33, Andrew Cooper wrote:
> On 21/10/2024 12:57 pm, Roger Pau Monné wrote:
>> On Mon, Oct 21, 2024 at 12:10:14PM +0100, Andrew Cooper wrote:
>>> On 18/10/2024 9:08 am, Roger Pau Monne wrote:
>>>> When using AMD-VI interrupt remapping the vector field in the IO-APIC RTE 
>>>> is
>>>> repurposed to contain part of the offset into the remapping table.  
>>>> Previous to
>>>> 2ca9fbd739b8 Xen had logic so that the offset into the interrupt remapping
>>>> table would match the vector.  Such logic was mandatory for end of 
>>>> interrupt to
>>>> work, since the vector field (even when not containing a vector) is used 
>>>> by the
>>>> IO-APIC to find for which pin the EOI must be performed.
>>>>
>>>> Introduce a table to store the EOI handlers when using interrupt 
>>>> remapping, so
>>>> that the IO-APIC driver can translate pins into EOI handlers without 
>>>> having to
>>>> read the IO-APIC RTE entry.  Note that to simplify the logic such table is 
>>>> used
>>>> unconditionally when interrupt remapping is enabled, even if strictly it 
>>>> would
>>>> only be required for AMD-Vi.
>>>>
>>>> Reported-by: Willi Junga <xenproject@xxxxxx>
>>>> Suggested-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>>>> Fixes: 2ca9fbd739b8 ('AMD IOMMU: allocate IRTE entries instead of using a 
>>>> static mapping')
>>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> Yet more fallout from the multi-MSI work.  That really has been a giant
>>> source of bugs.
>>>
>>>> ---
>>>>  xen/arch/x86/io_apic.c | 47 ++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 47 insertions(+)
>>>>
>>>> diff --git a/xen/arch/x86/io_apic.c b/xen/arch/x86/io_apic.c
>>>> index e40d2f7dbd75..8856eb29d275 100644
>>>> --- a/xen/arch/x86/io_apic.c
>>>> +++ b/xen/arch/x86/io_apic.c
>>>> @@ -71,6 +71,22 @@ static int apic_pin_2_gsi_irq(int apic, int pin);
>>>>  
>>>>  static vmask_t *__read_mostly vector_map[MAX_IO_APICS];
>>>>  
>>>> +/*
>>>> + * Store the EOI handle when using interrupt remapping.
>>>> + *
>>>> + * If using AMD-Vi interrupt remapping the IO-APIC redirection entry 
>>>> remapped
>>>> + * format repurposes the vector field to store the offset into the 
>>>> Interrupt
>>>> + * Remap table.  This causes directed EOI to longer work, as the CPU 
>>>> vector no
>>>> + * longer matches the contents of the RTE vector field.  Add a translation
>>>> + * table so that directed EOI uses the value in the RTE vector field when
>>>> + * interrupt remapping is enabled.
>>>> + *
>>>> + * Note Intel VT-d Xen code still stores the CPU vector in the RTE vector 
>>>> field
>>>> + * when using the remapped format, but use the translation table 
>>>> uniformly in
>>>> + * order to avoid extra logic to differentiate between VT-d and AMD-Vi.
>>>> + */
>>>> +static unsigned int **apic_pin_eoi;
>>> I think we can get away with this being uint8_t rather than unsigned
>>> int, especially as we're allocating memory when not strictly necessary.
>>>
>>> The only sentinel value we use is IRQ_VECTOR_UNASSIGNED which is -1.
>>>
>>> Vector 0xff is strictly SPIV and not allocated for anything else, so can
>>> be reused as a suitable sentinel here.
>> The coding style explicitly discourages using fixed width types unless
>> it's strictly necessary, I assume the usage here would be covered by
>> Xen caching a value of a hardware register field that has a
>> fixed-width size.

Use "short" then?

> I'm >< this close to reverting that too.  It's not even self-consistent
> as written, nonsense in some cases, and is being used as a whipping
> stick to reject otherwise-ok patches, which is pure toxicity in the
> community.

I don't really mind reverting that part, if only ...

> Not to mention that this rule is in contradiction to MISRA, and there's
> no progress being made in that direction.
> 
> 
> All variables should be of an appropriate type.
> 
> Sometimes that's a fixed width type, and sometimes it's not.  (And this
> is what is impossible to dictate in CODING_STYLE.)

... "appropriate" was definable in some way. Prior to this addition to
the style it was easily possible (and happening increasingly often, iirc)
for two people to disagree about what's appropriate, blocking progress.
I'm therefore also not really happy with you describing this as "dictate".

Jan