[edk2-devel] [PATCH v5 08/38] ArmPkg/ArmMmuLib: Avoid splitting block entries if possible

[Leif Lindholm]

On Tue, Mar 14, 2023 at 19:29:39 +0100, Ard Biesheuvel wrote:
> On Tue, 14 Mar 2023 at 19:13, Leif Lindholm <quic_llindhol at quicinc.com> wrote:
> >
> > On Mon, Mar 13, 2023 at 18:16:44 +0100, Ard Biesheuvel wrote:
> > > Currently, the ARM MMU page table logic will break down any block entry
> > > that overlaps with the region being mapped, even if the block entry in
> > > question is using the same attributes as the new region.
> > >
> > > This means that creating a non-executable mapping inside a region that
> > > is already mapped non-executable at a coarser granularity may trigger a
> > > call to AllocatePages (), which may recurse back into the page table
> > > code to update the attributes on the newly allocated page tables.
> > >
> > > Let's avoid this, by preserving the block entry if it already covers the
> > > region being mapped with the correct attributes.
> >
> > So if a later mapping is made inside the same block with conflicting
> > attributes? That triggers the break down at that point and because the
> > existing mapping did not conflict, it'll all flush out?
> >
> 
> Indeed.
> 
> The case here is simply, e.g., mapping a single page XP that is
> already covered by a 2 MB XP block: without this patch, we break down
> that 2 MB block into page mappings that all have the same attributes.
> If the 4k page being remapped is being allocated for a page table, we
> may end up with unbounded recursion.
> 
> If the attributes are actually different, the split still happens. But
> otherwise, the block mapping is retained.

Makes sense.

Reviewed-by: Leif Lindholm <quic_llindhol at quicinc.com>


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