On 29.07.20 11:35, Peng Fan wrote:
Subject: Re: jailhouse jitter?
On 29.07.20 05:28, Peng Fan wrote:
Subject: Re: jailhouse jitter?
On 27.07.20 08:56, Peng Fan wrote:
Subject: Re: jailhouse jitter?
On 27.07.20 08:25, Peng Fan wrote:
Hi Jan
...
I tested the SDEI on i.MX8MM, it shows the jitter became smaller.
Without SDEI, the gic-demo jitter is 999ns+ With SDEI, the
gic-demo jitter is 124ns~246ns.
Indeed no more vmexits.
But the max jitter, some times SDEI bigger only when program start
up,
mostly because of CACHE WARM UP I think.
That is one source. If you add a warm-up period, they can be
mitigated, though.
The other source might be last-level cache sharing. If there are
cache-miss counters, maybe you can check if those increase along
the
peaks.
Yes. When I add stress-ng in root cell, the jitter became larger sometimes.
I've seen the same on the ultra96. My cache theory should be
validated, though, because I would have assumed that all of the gic
demo fits into a core-local cache.
After thinking more about root cell color, when booting jailhouse before
Linux.
We are not using 1:1 mapping anymore or we could use 1:1 mapping with
many pieces of small ram area.
I suspect Linux may not like a device tree with hundreds or thousands of
memory region entries.
True.
So I think the first 1 is better, but when use kmalloc for dma usage
sometimes, it will bring issues, because not 1:1 mapping, unless we
let all drivers use dedicated dma area reserved and not colored.
We will need an SMMU for colored Linux instances. That will make things
appear 1:1 mapped again for Linux.
But SMMU is really eating silicon die size, many platforms not have that.
Well, then add proper cache partitioning in hardware. Then we can drop
all those coloring hacks. ;)
But without SMMU (or at least some IO-MPU), you won't get a proper
isolation, only collaborative partitioning.
Jan
--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux
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