Reji,
On 2021-12-08 02:36, Reiji Watanabe wrote:
Hi Alex,
On Mon, Dec 6, 2021 at 9:02 AM Alexandru Elisei
<alexandru.elisei@xxxxxxx> wrote:
(CC'ing Peter Maydell in case this might be of interest to qemu)
The series can be found on a branch at [1], and the kvmtool support at
[2].
The kvmtool patches are also on the mailing list [3] and haven't
changed
since v1.
Detailed explanation of the issue and symptoms that the patches
attempt to
correct can be found in the cover letter for v1 [4].
A brief summary of the problem is that on heterogeneous systems KVM
will
always use the same PMU for creating the VCPU events for *all* VCPUs
regardless of the physical CPU on which the VCPU is running, leading
to
events suddenly stopping and resuming in the guest as the VCPU thread
gets
migrated across different CPUs.
This series proposes to fix this behaviour by allowing the user to
specify
which physical PMU is used when creating the VCPU events needed for
guest
PMU emulation. When the PMU is set, KVM will refuse to the VCPU on a
physical which is not part of the supported CPUs for the specified
PMU.
Just to confirm, this series provides an API for userspace to request
KVM to detect a wrong affinity setting due to a userspace bug so that
userspace can get an error at KVM_RUN instead of leading to events
suddenly stopping, correct ?
More than that, it allows userspace to select which PMU will be used
for their guest. The affinity setting is a byproduct of the PMU's own
affinity.
The default behaviour stays the same - without userspace setting the
PMU,
events will stop counting if the VCPU is scheduled on the wrong CPU.
Can't we fix the default behavior (in addition to the current fix) ?
(Do we need to maintain the default behavior ??)
Of course we do. This is a behaviour that has been exposed to userspace
for years, and *we don't break userspace*.
IMHO I feel it is better to prevent userspace from configuring PMU
for guests on such heterogeneous systems rather than leading to
events suddenly stopping even as the default behavior.
People running KVM on asymmetric systems *strongly* disagree with you.
M.
--
Jazz is not dead. It just smells funny...
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