Re: [PATCH 2/2] arm64: mte: Defer disabling of TCO until user_access_begin/end

["Taehyun Noh" <taehyun@utexas.edu>]

Hi,

On Thu Jan 8, 2026 at 12:45 PM CST, Catalin Marinas wrote:
> Reading the Arm ARM section again, I wonder whether always setting TCMA1
> does the trick for the Ampere hardware. With KASAN disabled in the
> kernel, all addresses will star with 0xff... so behave as match-all. We
> do this with KASAN_HW_TAGS enabled but it won't have any effect with
> kasan disabled.

Our team agrees with Catalin’s TCMA1 solution. It disables every kernel
tag checking but the user address will get tag checked as far as TCO is
clear. Also, Carl’s initial testing confirms that
`mem_access_checked*:k` counters drop with the TCMA1 patch. While we
haven’t run the memcached benchmark yet, we will follow up with those
results shortly.

Additionally, we’ve observed that Pixel 9 behaves differently; the
kernel does not perform any tag checking when the user process enables
MTE. I’ve tested a simple kernel module that accesses kernel memory on
user ioctl, and measured the MTE perf counters on both AmpereOne and
Pixel 9. Pixel 9 shows no increases in checked access counters, but
AmpereOne shows proportional increases depending on the buffer size that
is accessed inside the kernel module.

We will keep you posted as more data becomes available.

Taehyun