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

H.J. Lu
2022-05-13T00:09:17Z

On Thu, May 12, 2022 at 4:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Thu, May 12 2022 at 15:10, H. J. Lu wrote:
> > 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.
>
> That's at least a halfways useful answer.
>
> > An application can ask for LAM_U48 or LAM_U57. But the decision should
> > be made by application.
>
> It can ask for whatever, but the decision whether it's granted is made
> by the kernel for obvious reasons.
>
> > When an application asks for LAM_U57, I expect it will store tags in
> > upper 6 bits, even if the kernel enables LAM_U48.
>
> The kernel does not enable LAM_U48 when the application only wants to
> have LAM_U57, because that would restrict the address space of the
> application to 47 bits on 5-level capable system for no reason.
>
> So what are you trying to tell me?
>

I am expecting applications to ask for LAM_U48 or LAM_U57, not just
LAM.

--
H.J.