Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest
From: Like Xu
Date: Tue Jan 26 2021 - 15:56:25 EST
Hi Peter,
On 2021/1/15 22:44, Peter Zijlstra wrote:
On Fri, Jan 15, 2021 at 10:30:13PM +0800, Xu, Like wrote:
Are you sure? Spurious NMI/PMIs are known to happen anyway. We have far
too much code to deal with them.
https://lore.kernel.org/lkml/20170628130748.GI5981@leverpostej/T/
In the rr workload, the commit change "the PMI interrupts in skid region
should be dropped"
is reverted since some users complain that:
It seems to me that it might be reasonable to ignore the interrupt if
the purpose of the interrupt is to trigger sampling of the CPUs
register state. But if the interrupt will trigger some other
operation, such as a signal on an fd, then there's no reason to drop
it.
I assume that if the PMI drop is unacceptable, either will spurious PMI
injection.
I'm pretty open if you insist that we really need to do this for guest PEBS
enabling.
That was an entirely different issue. We were dropping events on the
floor because they'd passed priv boundaries. So there was an actual
event, and we made it go away.
What we're talking about here is raising an PMI with BUFFER_OVF set,
even if the DS is empty. That should really be harmless. We'll take the
PMI, find there's nothing there, and do nothing.
In the host and guest PEBS both enabled case,
we'll get a crazy dmesg *bombing* about spurious PMI warning
if we pass the host PEBS PMI "harmlessly" to the guest:
[11261.502536] Uhhuh. NMI received for unknown reason 2c on CPU 36.
[11261.502539] Do you have a strange power saving mode enabled?
[11261.502541] Dazed and confused, but trying to continue
Legacy guest users may be very confused and dissatisfied with that.
I'm double checking with you if it's acceptable to take the proposal
"disables the co-existence of guest PEBS and host PEBS" as the first
step to upstream, and enable both host and guest PEBS in the near future.
---
thx,likexu