On Tue, 04 Apr 2023 20:43:40 +0100, Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > Folks, > > On Thu, Mar 30, 2023 at 03:43:35PM -0700, David Dai wrote: > > <snip> > > > PCMark > > Higher is better > > +-------------------+----------+------------+--------+-------+--------+ > > | Test Case (score) | Baseline | Hypercall | %delta | MMIO | %delta | > > +-------------------+----------+------------+--------+-------+--------+ > > | Weighted Total | 6136 | 7274 | +19% | 6867 | +12% | > > +-------------------+----------+------------+--------+-------+--------+ > > | Web Browsing | 5558 | 6273 | +13% | 6035 | +9% | > > +-------------------+----------+------------+--------+-------+--------+ > > | Video Editing | 4921 | 5221 | +6% | 5167 | +5% | > > +-------------------+----------+------------+--------+-------+--------+ > > | Writing | 6864 | 8825 | +29% | 8529 | +24% | > > +-------------------+----------+------------+--------+-------+--------+ > > | Photo Editing | 7983 | 11593 | +45% | 10812 | +35% | > > +-------------------+----------+------------+--------+-------+--------+ > > | Data Manipulation | 5814 | 6081 | +5% | 5327 | -8% | > > +-------------------+----------+------------+--------+-------+--------+ > > > > PCMark Performance/mAh > > Higher is better > > +-----------+----------+-----------+--------+------+--------+ > > | | Baseline | Hypercall | %delta | MMIO | %delta | > > +-----------+----------+-----------+--------+------+--------+ > > | Score/mAh | 79 | 88 | +11% | 83 | +7% | > > +-----------+----------+-----------+--------+------+--------+ > > > > Roblox > > Higher is better > > +-----+----------+------------+--------+-------+--------+ > > | | Baseline | Hypercall | %delta | MMIO | %delta | > > +-----+----------+------------+--------+-------+--------+ > > | FPS | 18.25 | 28.66 | +57% | 24.06 | +32% | > > +-----+----------+------------+--------+-------+--------+ > > > > Roblox Frames/mAh > > Higher is better > > +------------+----------+------------+--------+--------+--------+ > > | | Baseline | Hypercall | %delta | MMIO | %delta | > > +------------+----------+------------+--------+--------+--------+ > > | Frames/mAh | 91.25 | 114.64 | +26% | 103.11 | +13% | > > +------------+----------+------------+--------+--------+--------+ > > </snip> > > > Next steps: > > =========== > > We are continuing to look into communication mechanisms other than > > hypercalls that are just as/more efficient and avoid switching into the VMM > > userspace. Any inputs in this regard are greatly appreciated. > > We're highly unlikely to entertain such an interface in KVM. > > The entire feature is dependent on pinning vCPUs to physical cores, for which > userspace is in the driver's seat. That is a well established and documented > policy which can be seen in the way we handle heterogeneous systems and > vPMU. > > Additionally, this bloats the KVM PV ABI with highly VMM-dependent interfaces > that I would not expect to benefit the typical user of KVM. > > Based on the data above, it would appear that the userspace implementation is > in the same neighborhood as a KVM-based implementation, which only further > weakens the case for moving this into the kernel. > > I certainly can appreciate the motivation for the series, but this feature > should be in userspace as some form of a virtual device. +1 on all of the above. The one thing I'd like to understand that the comment seems to imply that there is a significant difference in overhead between a hypercall and an MMIO. In my experience, both are pretty similar in cost for a handling location (both in userspace or both in the kernel). MMIO handling is a tiny bit more expensive due to a guaranteed TLB miss followed by a walk of the in-kernel device ranges, but that's all. It should hardly register. And if you really want some super-low latency, low overhead signalling, maybe an exception is the wrong tool for the job. Shared memory communication could be more appropriate. Thanks, M. -- Without deviation from the norm, progress is not possible.
