[0/2] accel/tcg: remove implied BQL from cpu_handle_interrupt/exception path

Message

Robert Foley July 31, 2020, 12:51 p.m. UTC

The purpose of this change is to set the groundwork
so that an arch could move towards removing
the BQL from the cpu_handle_interrupt/exception paths.

The BQL is a bottlneck in scaling to more cores.
And this cpu_handle_interrupt/exception path is one of
the key BQL users as measured by the QEMU sync profiling (qsp).

We have chosen to break up the process of removing
BQL from this path into two pieces:

1) Changes to the core/common functions
   of cpu_handle_interrupt/exception
   allowing a per-arch decision to hold BQL.
   The common case and the default is for the core code
   to hold the BQL (bql=true).
   This set of changes is handled in this patch.

2) Removing the BQL from the per-arch functions.
   1) makes it possible for an arch to set bql=false
   so that the common code does not hold the BQL
   across the cpu_handle_interrupt/exception call.
   This allows the arch to handle locking in this path
   We leave it up to the arch to make the change
   at the time that makes sense.

It is worth mentioning that we are working on per-arch changes
in line with 2), and plan to submit these.
In other words, we plan to set the groundwork with this
patch series and then will take advantage of it in later series.

This patch series is based on the per-CPU locks patch:
https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg05314.html

Robert Foley (2):
  hw/core: Add bql_interrupt flag to CPUClass
  accel/tcg:  interrupt/exception handling uses bql_interrupt flag

 accel/tcg/cpu-exec.c  | 34 ++++++++++++++++++++++++++--------
 hw/core/cpu.c         |  1 +
 include/hw/core/cpu.h |  8 ++++++++
 3 files changed, 35 insertions(+), 8 deletions(-)