Re: [GIT PULL] arm64: Fixes for -rc4

[Jayachandran Chandrasekharan Nair]

On Fri, Oct 18, 2019 at 12:43:49AM +0100, Will Deacon wrote:
> Hi Linus,
> 
> Please pull these arm64 fixes for -rc4. The main thing here is a
> long-awaited workaround for a CPU erratum on ThunderX2 which we have
> developed in conjunction with engineers from Cavium/Marvell. At the moment,
> the workaround is unconditionally enabled for affected CPUs at runtime
> but we may add a command-line option to disable it in future if performance
> numbers show up indicating a significant cost for real workloads.

As the Cavium/Marvell engineer who was involved in this, I will note
that I had suggested a patch providing a runtime override[1] while
providing safe defaults.

Marc's patchset adds a trap to hypervisor in the system call path when
KPTI is enabled, and KPTI is generally enabled on stock VM images. So
normal users will see some performance regression (e.g I see something
in the range of 3-4% on guest kernel compile).

As a policy, I don't agree with having errata workarounds that can be
left to the discretion of the admin to be forced at compile time.
Since most of these workarounds use run-time code patching with
alternatives, there is no need to do this at all.

But given that this is already merged and cc:ed to stable, I will see
if I can come up with enough data to convince Will.

JC
[1]
https://lore.kernel.org/linux-arm-kernel/20191011232031.GA29752@xxxxxxxxxxxxxxxxxxxxxxxx/T/