Re: [PATCH 2/5] ArmPkg/ArmMmuLib AARCH64: get rid of needless TLB invalidation

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Mon, 28 Jan 2019 at 19:01, Leif Lindholm <leif.lindholm@linaro.org> wrote:
>
> On Mon, Jan 28, 2019 at 01:29:54PM +0100, Ard Biesheuvel wrote:
> > > > > > > > @@ -296,7 +297,8 @@ GetBlockEntryListFromAddress (
> > > > > > > >
> > > > > > > >          // Fill the BlockEntry with the new TranslationTable
> > > > > > > >          ReplaceLiveEntry (BlockEntry,
> > > > > > > > -          ((UINTN)TranslationTable & TT_ADDRESS_MASK_DESCRIPTION_TABLE) | TableAttributes | TT_TYPE_TABLE_ENTRY);
> > > > > > > > +          (UINTN)TranslationTable | TableAttributes | TT_TYPE_TABLE_ENTRY,
> > > > > > > > +          RegionStart);
> > > > > > >
> > > > > >
> > > > > > /me pages in the data ...
> > > > > >
> > > > > > > OK, this whole patch took a few times around the loop before I think I
> > > > > > > caught on what was happening.
> > > > > > >
> > > > > > > I think I'm down to the only things confusing me being:
> > > > > > > - The name "Address" to refer to something that is always the start
> > > > > > >   address of a 4KB-aligned translation region.
> > > > > > >   Is this because the function will be usable in other contexts in
> > > > > > >   later patches?
> > > > > >
> > > > > > I could change it to VirtualAddress if you prefer.
> > > > > > It is only passed
> > > > > > for TLB maintenance which is only needed at page granularity, and the
> > > > > > low bits are shifted out anyway.
> > > > >
> > > > > Yeah, exactly. It would just be nice if the name reflected that. Not
> > > > > sure VirtualAddress does. VirtualBase? PageBase?
> > > > >
> > > >
> > > > Well, the argument passed in is called RegionStart, so let's just
> > > > stick with that.
> > >
> > > Sure. With that:
> > > Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
> > >
> >
> > Thanks
> >
> > GIven the discussion in the other thread regarding shareability
> > upgrades of barriers and TLB maintenance instructions when running
> > under virt, mind if I fold in the hunk below? (and add a mention to
> > the commit log)
> >
> > --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S
> > +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S
> > @@ -35,7 +35,7 @@
> >    // flush translations for the target address from the TLBs
> >    lsr   x2, x2, #12
> >    tlbi  vae\el, x2
> > -  dsb   sy
> > +  dsb   nsh
>
> So, this one, definitely. MMU is off, shareability is a shady concept
> at best, so it's arguably a fix.
>
> >    // re-enable the MMU
> >    msr   sctlr_el\el, x8
> > @@ -58,7 +58,7 @@ ASM_FUNC(ArmReplaceLiveTranslationEntry)
> >    // clean and invalidate first so that we don't clobber
> >    // adjacent entries that are dirty in the caches
> >    dc    civac, x0
> > -  dsb   ish
> > +  dsb   nsh
>
> This one I guess is safe because we know we're never going to get
> pre-empted or migrated (because interrupts are disabled and we don't
> do that sort of thing)?
>

Well, we can only get pre-empted under virt, in which case HCR_EL2.BSU
will be set so that barriers are upgraded to inner-shareable. In bare
metal cases, we only care about the local CPU since there are no other
masters (nor DMA capable devices) that care about this cache
maintenance having completed since the page tables are used by the CPU
only.

> If that's the rationale:
> Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
>

Thanks

Pushed as f34b38fae614..d5788777bcc7