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

[Leif Lindholm <leif.lindholm@linaro.org>]

On Wed, Jan 23, 2019 at 04:55:28PM +0100, Ard Biesheuvel wrote:
> On Wed, 23 Jan 2019 at 16:46, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> >
> > On Mon, Jan 07, 2019 at 08:15:01AM +0100, Ard Biesheuvel wrote:
> > > Currently, we always invalidate the TLBs entirely after making
> > > any modification to the page tables. Now that we have introduced
> > > strict memory permissions in quite a number of places, such
> > > modifications occur much more often, and it is better for performance
> > > to flush only those TLB entries that are actually affected by
> > > the changes.
> > >
> > > Contributed-under: TianoCore Contribution Agreement 1.1
> > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > > ---
> > >  ArmPkg/Include/Library/ArmMmuLib.h                       |  3 ++-
> > >  ArmPkg/Library/ArmLib/AArch64/ArmLibSupport.S            |  6 +++---
> > >  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c         | 16 +++++++---------
> > >  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibReplaceEntry.S | 14 ++++++++------
> > >  4 files changed, 20 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > > index d66df3e17a02..e1fabfcbea14 100644
> > > --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > > +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> > > @@ -129,13 +129,14 @@ STATIC
> > >  VOID
> > >  ReplaceLiveEntry (
> > >    IN  UINT64  *Entry,
> > > -  IN  UINT64  Value
> > > +  IN  UINT64  Value,
> > > +  IN  UINT64  Address
> > >    )
> > >  {
> > >    if (!ArmMmuEnabled ()) {
> > >      *Entry = Value;
> > >    } else {
> > > -    ArmReplaceLiveTranslationEntry (Entry, Value);
> > > +    ArmReplaceLiveTranslationEntry (Entry, Value, Address);
> > >    }
> > >  }
> > >
> > > @@ -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?

> > - Why drop the & TT_ADDRESS_MASK_DESCRIPTION_TABLE bit here?
> >   Was it just always pointless and you decided to drop it while you
> >   were at it?
> 
> IIRC yes. It is a newly allocated page, so the masking does not do anything.

Yeah, that's fair enough.
Just made me wonder if anything unobvious was going on :)

/
    Leif