Re: [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch

[Dan Williams]

On Fri, Dec 18, 2020 at 1:06 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 18 2020 at 11:20, Dan Williams wrote:
> > On Fri, Dec 18, 2020 at 5:58 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > [..]
> >>   5) The DAX case which you made "work" with dev_access_enable() and
> >>      dev_access_disable(), i.e. with yet another lazy approach of
> >>      avoiding to change a handful of usage sites.
> >>
> >>      The use cases are strictly context local which means the global
> >>      magic is not used at all. Why does it exist in the first place?
> >>
> >>      Aside of that this global thing would never work at all because the
> >>      refcounting is per thread and not global.
> >>
> >>      So that DAX use case is just a matter of:
> >>
> >>         grant/revoke_access(DEV_PKS_KEY, READ/WRITE)
> >>
> >>      which is effective for the current execution context and really
> >>      wants to be a distinct READ/WRITE protection and not the magic
> >>      global thing which just has on/off. All usage sites know whether
> >>      they want to read or write.
> >
> > I was tracking and nodding until this point. Yes, kill the global /
> > kmap() support, but if grant/revoke_access is not integrated behind
> > kmap_{local,atomic}() then it's not a "handful" of sites that need to
> > be instrumented it's 100s. Are you suggesting that "relaxed" mode
> > enforcement is a way to distribute the work of teaching driver writers
> > that they need to incorporate explicit grant/revoke-read/write in
> > addition to kmap? The entire reason PTE_DEVMAP exists was to allow
> > get_user_pages() for PMEM and not require every downstream-GUP code
> > path to specifically consider whether it was talking to PMEM or RAM
> > pages, and certainly not whether they were reading or writing to it.
>
> kmap_local() is fine. That can work automatically because it's strict
> local to the context which does the mapping.
>
> kmap() is dubious because it's a 'global' mapping as dictated per
> HIGHMEM. So doing the RELAXED mode for kmap() is sensible I think to
> identify cases where the mapped address is really handed to a different
> execution context. We want to see those cases and analyse whether this
> can't be solved in a different way. That's why I suggested to do a
> warning in that case.
>
> Also vs. the DAX use case I really meant the code in fs/dax and
> drivers/dax/ itself which is handling this via dax_read_[un]lock.
>
> Does that make more sense?

Yup, got it. The dax code can be precise wrt to PKS in a way that
kmap_local() cannot.