Re: [PATCHv8 00/11] Linear Address Masking enabling

[Peter Zijlstra]

On Mon, Sep 05, 2022 at 04:44:57PM +0300, Kirill A. Shutemov wrote:
> On Mon, Sep 05, 2022 at 10:35:44AM +0530, Bharata B Rao wrote:
> > Hi Kirill,
> > 
> > On 9/4/2022 6:30 AM, Kirill A. Shutemov wrote:
> > > On Tue, Aug 30, 2022 at 04:00:53AM +0300, Kirill A. Shutemov wrote:
> > >> Linear Address Masking[1] (LAM) modifies the checking that is applied to
> > >> 64-bit linear addresses, allowing software to use of the untranslated
> > >> address bits for metadata.
> > >>
> > >> The patchset brings support for LAM for userspace addresses. Only LAM_U57 at
> > >> this time.
> > >>
> > >> Please review and consider applying.
> > >>
> > >> git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git lam
> > > 
> > > +Bharata, Ananth.
> > > 
> > > Do you folks have any feedback on the patchset?
> > > 
> > > Looks like AMD version of the tagged pointers feature does not get
> > > traction as of now, but I want to be sure that the interface introduced
> > > here can be suitable for your future plans.
> > > 
> > > Do you see anything in the interface that can prevent it to be extended to
> > > the AMD feature?
> > 
> > The arch_prctl() extensions is generic enough that it should be good.
> > 
> > The untagged_addr() macro looks like this from one of the callers:
> > 
> >         start = untagged_addr(mm, start);
> > ffffffff814d39bb:       48 8b 8d 40 ff ff ff    mov    -0xc0(%rbp),%rcx
> > ffffffff814d39c2:       48 89 f2                mov    %rsi,%rdx
> > ffffffff814d39c5:       48 c1 fa 3f             sar    $0x3f,%rdx
> > ffffffff814d39c9:       48 0b 91 50 03 00 00    or     0x350(%rcx),%rdx
> > ffffffff814d39d0:       48 21 f2                and    %rsi,%rdx
> > ffffffff814d39d3:       49 89 d6                mov    %rdx,%r14
> > 
> > Can this overhead of a few additional instructions be removed for
> > platforms that don't have LAM feature? I haven't measured how much
> > overhead this effectively contributes to in real, but wonder if it is
> > worth optimizing for non-LAM platforms.
> 
> I'm not sure how the optimization should look like. I guess we can stick
> static_cpu_has() there, but I'm not convinced that adding jumps there will
> be beneficial.

I suppose the critical bit is the memory load. That can stall and then
you're sad. A jump_label is easy enough to add.