On 2021/5/18 16:47, Peter Zijlstra wrote:
On Mon, May 17, 2021 at 07:44:15AM -0700, Andi Kleen wrote:
On 5/17/2021 1:39 AM, Peter Zijlstra wrote:
On Tue, May 11, 2021 at 10:42:05AM +0800, Like Xu wrote:
+ if (pebs) {
+ /*
+ * The non-zero precision level of guest event makes the ordinary
+ * guest event becomes a guest PEBS event and triggers the host
+ * PEBS PMI handler to determine whether the PEBS overflow PMI
+ * comes from the host counters or the guest.
+ *
+ * For most PEBS hardware events, the difference in the software
+ * precision levels of guest and host PEBS events will not affect
+ * the accuracy of the PEBS profiling result, because the "event IP"
+ * in the PEBS record is calibrated on the guest side.
+ */
+ attr.precise_ip = 1;
+ }
You've just destroyed precdist, no?
precdist can mean multiple things:
- Convert cycles to the precise INST_RETIRED event. That is not meaningful
for virtualization because "cycles" doesn't exist, just the raw events.
- For GLC+ and TNT+ it will force the event to a specific counter that is
more precise. This would be indeed "destroyed", but right now the patch kit
only supports Icelake which doesn't support that anyways.
So I think the code is correct for now, but will need to be changed for
later CPUs. Should perhaps fix the comment though to discuss this.
OK, can we then do a better comment that explains *why* this is correct
now and what needs help later?
Because IIUC the only reason it is correct now is because:
- we only support ICL
* and ICL has pebs_format>=2, so {1,2} are the same
* and ICL doesn't have precise_ip==3 support
- Other hardware (GLC+, TNT+) that could possibly care here
is unsupported atm. but needs changes.
None of which is actually mentioned in that comment it does have.
Hi Andi & Peter,
By "precdist", do you mean the"Precise Distribution of Instructions Retired
(PDIR) Facility"?
The SDM says Ice Lake Microarchitecture does support PEBS-PDIR on
IA32_FIXED0 only.
And this patch kit enables it in the patch 0011, please take a look.
Or do I miss something about precdist on ICL ?
Thanks,
Like Xu
