Replace the magic vector allocation code by a simple bitmap matrix
allocator. This avoids loops and hoops over CPUs and vector arrays, so in
case of densly used vector spaces it's way faster.
This also gets rid of the magic 'spread the vectors accross priority
levels' heuristics in the current allocator:
The comment in __asign_irq_vector says:
* NOTE! The local APIC isn't very good at handling
* multiple interrupts at the same interrupt level.
* As the interrupt level is determined by taking the
* vector number and shifting that right by 4, we
* want to spread these out a bit so that they don't
* all fall in the same interrupt level.
After doing some palaeontological research the following was found the
following in the PPro Developer Manual Volume 3:
"7.4.2. Valid Interrupts
The local and I/O APICs support 240 distinct vectors in the range of 16
to 255. Interrupt priority is implied by its vector, according to the
following relationship: priority = vector / 16
One is the lowest priority and 15 is the highest. Vectors 16 through
31 are reserved for exclusive use by the processor. The remaining
vectors are for general use. The processor's local APIC includes an
in-service entry and a holding entry for each priority level. To avoid
losing inter- rupts, software should allocate no more than 2 interrupt
vectors per priority."
The current SDM tells nothing about that, instead it states:
"If more than one interrupt is generated with the same vector number,
the local APIC can set the bit for the vector both in the IRR and the
ISR. This means that for the Pentium 4 and Intel Xeon processors, the
IRR and ISR can queue two interrupts for each interrupt vector: one
in the IRR and one in the ISR. Any additional interrupts issued for
the same interrupt vector are collapsed into the single bit in the
IRR.
For the P6 family and Pentium processors, the IRR and ISR registers
can queue no more than two interrupts per interrupt vector and will
reject other interrupts that are received within the same vector."
Which means, that on P6/Pentium the APIC will reject a new message and
tell the sender to retry, which increases the load on the APIC bus and
nothing more.
There is no affirmative answer from Intel on that, but it's a sane approach
to remove that for the following reasons:
1) No other (relevant Open Source) operating systems bothers to
implement this or mentiones this at all.
2) The current allocator has no enforcement for this and especially the
legacy interrupts, which are the main source of interrupts on these
P6 and older systmes, are allocated linearly in the same priority
level and just work.
3) The current machines have no problem with that at all as verified
with some experiments.
4) AMD at least confirmed that such an issue is unknown.
5) P6 and older are dinosaurs almost 20 years EOL, so there is really
no reason to worry about that too much.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juergen Gross <jgross@suse.com>
Tested-by: Yu Chen <yu.c.chen@intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Rui Zhang <rui.zhang@intel.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Len Brown <lenb@kernel.org>
Link: https://lkml.kernel.org/r/20170913213155.443678104@linutronix.de
69cde0004a