Re: [RFCv2 00/10] Linear Address Masking enabling

H.J. Lu
2022-05-12T22:11:03Z

On Thu, May 12, 2022 at 2:51 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 5/12/22 12:39, Thomas Gleixner wrote:
> >> It's OK for a debugging build that runs on one kind of hardware. But,
> >> if we want LAM-using binaries to be portable, we have to do something
> >> different.
> >>
> >> One of the stated reasons for adding LAM hardware is that folks want to
> >> use sanitizers outside of debugging environments. To me, that means
> >> that LAM is something that the same binary might run with or without.
> > On/off yes, but is there an actual use case where such a mechanism would
> > at start time dynamically chose the number of bits?
>
> I'd love to hear from folks doing the userspace side of this. Will
> userspace be saying: "Give me all the bits you can!". Or, will it
> really just be looking for 6 bits only, and it doesn't care whether it
> gets 6 or 15, it will use only 6?
>
> Do the sanitizers have more overhead with more bits? Or *less* overhead
> because they can store more metadata in the pointers?
>
> Will anyone care about the difference about potentially missing 1/64
> issues with U57 versus 1/32768 with U48?

The only LAM usage I know so far is LAM_U57 in HWASAN. An application
can ask for LAM_U48 or LAM_U57. But the decision should be made by
application. When an application asks for LAM_U57, I expect it will store
tags in upper 6 bits, even if the kernel enables LAM_U48.

--
H.J.