On 01/04/2019 11:57 PM, Liang, Kan wrote:
On 1/4/2019 4:58 AM, Wei Wang wrote:
On 01/03/2019 12:33 AM, Liang, Kan wrote:
On 12/26/2018 4:25 AM, Wei Wang wrote:
+
+ /*
+ * It could be possible that people have vcpus of old model
run on
+ * physcal cpus of newer model, for example a BDW guest on a SKX
+ * machine (but not possible to be the other way around).
+ * The BDW guest may not get accurate results on a SKX machine
as it
+ * only reads 16 entries of the lbr stack while there are 32
entries
+ * of recordings. So we currently forbid the lbr enabling when
the
+ * vcpu and physical cpu see different lbr stack entries.
I think it's not enough to only check number of entries. The LBR
from/to MSRs may be different even the number of entries is the
same, e.g SLM and KNL.
Yes, we could add the comparison of the FROM msrs.
+ */
+ switch (vcpu_model) {
That's a duplicate of intel_pmu_init(). I think it's better to
factor out the common part if you want to check LBR MSRs and
entries. Then we don't need to add the same codes in two different
places when enabling new platforms.
Yes, I thought about this, but intel_pmu_init() does a lot more
things in each "Case xx". Any thought about how to factor them out?
I think we may only move the "switch (boot_cpu_data.x86_model) { ...
}" to a new function, e.g. __intel_pmu_init(int model, struct x86_pmu
*x86_pmu)
In __intel_pmu_init, if the model != boot_cpu_data.x86_model, you only
need to update x86_pmu.*. Just ignore global settings, e.g
hw_cache_event_ids, mem_attr, extra_attr etc.
Thanks for sharing. I understand the point of maintaining those models
at one place,
but this factor-out doesn't seem very elegant to me, like below
__intel_pmu_init (int model, struct x86_pmu *x86_pmu)
{
...
switch (model)
case INTEL_FAM6_NEHALEM:
case INTEL_FAM6_NEHALEM_EP:
case INTEL_FAM6_NEHALEM_EX:
intel_pmu_lbr_init(x86_pmu);
if (model != boot_cpu_data.x86_model)
return;
/* Other a lot of things init like below..*/
memcpy(hw_cache_event_ids, nehalem_hw_cache_event_ids,
sizeof(hw_cache_event_ids));
memcpy(hw_cache_extra_regs, nehalem_hw_cache_extra_regs,
sizeof(hw_cache_extra_regs));
x86_pmu.event_constraints = intel_nehalem_event_constraints;
x86_pmu.pebs_constraints =
intel_nehalem_pebs_event_constraints;
x86_pmu.enable_all = intel_pmu_nhm_enable_all;
x86_pmu.extra_regs = intel_nehalem_extra_regs;
...
Case...
}
We need insert "if (model != boot_cpu_data.x86_model)" in every "Case xx".
What would be the rationale that we only do lbr_init for "x86_pmu"
when model != boot_cpu_data.x86_model?
(It looks more like a workaround to factor-out the function and get what
we want)
I would prefer having them separated as this patch for now - it is
logically more clear to me.
Actually, I think we may just support LBR for guest if it has the
identical CPU model as host. It should be good enough for now.
I actually tried this in the first place but it failed to work with
the existing QEMU.
For example, when we specify "Broadwell" cpu from qemu, then qemu
uses Broadwell core model,
but the physical machine I have is Broadwell X. This patch will
support this case.
I mean is it good enough if we only support "-cpu host"?
Not really. AFAIK, people don't use this usually. It is more common to
specify the CPU type.
Best,
Wei
