[PATCH V4 2/2] arm: mm: Switch back to L_PTE_WRITE

[steve.capper@linaro.org (Steve Capper)]

On Fri, Jun 20, 2014 at 07:17:48PM +0100, Russell King - ARM Linux wrote:
> On Mon, Jun 16, 2014 at 03:32:39PM +0100, Steve Capper wrote:
> > For LPAE, we have the following means for encoding writable or dirty
> > ptes:
> >                               L_PTE_DIRTY       L_PTE_RDONLY
> >     !pte_dirty && !pte_write        0               1
> >     !pte_dirty && pte_write         0               1
> >     pte_dirty && !pte_write         1               1
> >     pte_dirty && pte_write          1               0
> > 
> > So we can't distinguish between writable clean ptes and read only
> > ptes. This can cause problems with ptes being incorrectly flagged as
> > read only when they are writable but not dirty.
> > 
> > This patch re-introduces the L_PTE_WRITE bit for both short descriptors
> > and long descriptors, by reverting
> >   36bb94b ARM: pgtable: provide RDONLY page table bit rather than WRITE bit
> 
> Why are we still going about this in this over complicated manner?
> I'm not happy with this.  I thought after fixing the problem with
> using bits above bit 32 that we could drop this silly conversion
> which makes the code harder to read.
> 
> Right, let's get down to the detail.  LPAE has it's existing bit
> which tells it that the mapping is read only.  This is bit 7, which
> is the AP[2] bit.
> 
> At present, AP[2] is mapped to L_PTE_RDONLY.  When a PTE is set, the
> 3-level page table code in proc-v7-3level.S checks the L_PTE_DIRTY
> bit, and if that is clear, it sets L_PTE_RDONLY.  *This* is the
> problem you're trying to solve.
> 
> You are solving that by adding L_PTE_WRITE as bit 58 on LPAE, and
> then translating bit 58 _and_ the L_PTE_DIRTY state down to a
> read-only status for the hardware in AP[2], and rolling the change
> to make L_PTE_WRITE apply everywhere.
> 
> Now, in patch 1, we solve the problem that using high bits in the
> PTE result in the return value being down-cast to zero.  So, with
> patch 1 in place, we can use *any* bit in the PTE to correspond
> with any of the L_PTE_* flags.  Remember this very important point:
> L_PTE_* flags are the *Linux* representation of the page table state,
> which may not necessarily be the state of the hardware (it isn't on
> 2-level systems - there's a translation that this stuff goes through.)
> 
> So, what I say is why not, for the troublesome 3-level case:
> 
> - Assign bit 58 for L_PTE_RDONLY
> - Convert the state of bit 58 and L_PTE_DIRTY to the AP[2] bit:
> 
> 	ubfx	ip, rh, #(58 - 32)		@ L_PTE_RDONLY
> 	bfi	rl, ip, #7, #1			@ PTE_AP2
>         tst     rh, #1 << (55 - 32)             @ L_PTE_DIRTY
>         orreq   rl, #PTE_AP2
> 
> This means we keep the read-only terminology, which is much more
> understandable when reading the assembly code than what we had when
> we used the write terminology.

Hi Russell,
Thanks for the advice, yes segregating L_PTE_RDONLY from PTE_AP2 allows
for a much smaller patch that leaves 2-level alone.

I am running a barrage of tests on a new series now that follows this
logic and will post a new revision soon.

Cheers,
-- 
Steve