Re: [PATCH v4 0/6] Add AutoFDO and Propeller support for Clang build

[Rong Xu <xur@google.com>]

On Sat, Oct 19, 2024 at 8:25 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Hi Rong,
>
> On Fri, Oct 18, 2024 at 11:20:02PM -0700, Rong Xu wrote:
> > Thanks to all for the feedback and suggestions! We are ready to make any further
> > changes needed. Is there anything else we can address for this patch?
>
> I will reply in a separate thread for visibility but I think one of the
> biggest open questions at the moment is trying to find someone to
> shepherd this code into mainline.
>
> > Also, we know it's not easy to test this patch, but if anyone has had a chance
> > to try building AutoFDO/Propeller kernels with it, we'd really appreciate your
> > input here. Any confirmation that it works as expected would be very helpful.
>
> I went to take this series for a spin in a virtual machine first as a
> smoke test before attempting to boot on bare metal. This was done on a
> server with an Intel Xeon Gold 6314U. The kernel booted fine but when I
> went to run the command to generate the perf data from the
> documentation, I get an error.
>
>   $ perf record -e BR_INST_RETIRED.NEAR_TAKEN:k -a -N -b -c 500009 -o /tmp/perf.data -- make -j$(nproc) O=out mrproper defconfig all
>   Error:
>   BR_INST_RETIRED.NEAR_TAKEN:k: PMU Hardware or event type doesn't support branch stack sampling.
>
> Do you know if this is expected for a virtual machine setup? I will
> attempt to test the series on real hardware here soon, it is currently
> tied up with investigating a regression in -next at the moment.

We have never tested this patch in a KVM setup.

As far as we know, LBR support in KVM is currently limited, and varies
depending on the PMU virtualization model:
(1) For legacy mode, LBR profiling might work under LBR virtualization
(VLBR). However, we have not tested this.
(2) For the new "Mediated vPMU passthru' mode, there is no LBR
virtualization support at this point. So LBR profiling is not working.

I've included Stephance here. He should have more expertise on this topic.

>
> Cheers,
> Nathan