Re: [Xen-devel] [PATCH RFC] arm64: 32-bit tolerant sync bitops

[Vladimir Murzin <murzin.v@xxxxxxxxx>]

On Wed, Apr 23, 2014 at 09:31:29AM +0100, Ian Campbell wrote:
> On Thu, 2014-04-17 at 11:42 +0100, David Vrabel wrote:
> > On 17/04/14 09:38, Vladimir Murzin wrote:
> > > Xen assumes that bit operations are able to operate on 32-bit size and
> > > alignment [1]. For arm64 bitops are based on atomic exclusive load/store
> > > instructions to guarantee that changes are made atomically. However, these
> > > instructions require that address to be aligned to the data size. 
> > > Because, by
> > > default, bitops operates on 64-bit size it implies that address should be
> > > aligned appropriately. All these lead to breakage of Xen assumption for 
> > > bitops
> > > properties.
> > > 
> > > With this patch 32-bit sized/aligned bitops is implemented. 
> > > 
> > > [1] http://www.gossamer-threads.com/lists/xen/devel/325613
> > > 
> > > Signed-off-by: Vladimir Murzin <murzin.v@xxxxxxxxx>
> > > ---
> > >  Apart this patch other approaches were implemented:
> > >  1. turn bitops to be 32-bit size/align tolerant.
> > >     the changes are minimal, but I'm not sure how broad side effect might 
> > > be
> > >  2. separate 32-bit size/aligned operations.
> > >     it exports new API, which might not be good
> > 
> > I've never been particularly happy with the way the events_fifo.c uses
> > casts for the sync_*_bit() calls and I think we should do option 2.
> > 
> > A generic implementation could be something like:
> 
> It seems like there isn't currently much call for/interest in a generic
> 32-bit set of bitops. Since this is a regression on arm64 right now how
> about we simply fix it up in the Xen layer now and if in the future a
> common need is found we can rework to use it.
> 
> We already have evtchn_fifo_{clear,set,is}_pending (although
> evtchn_fifo_unmask and consume_one_event need fixing to use is_pending)
> and evtchn_fifo_{test_and_set_,}mask, plus we would need
> evtchn_fifo_set_mask for consume_one_event to use instead of open
> coding.
> 
> With that then those helpers can become something like:
>         #define BM(w) (unsigned long *)(w & ~0x7)
>         #define EVTCHN_FIFO_BIT(b, w) (BUG_ON(w&0x3), w & 0x4 ? 
> EVTCHN_FIFO_#b + 32 : EVTCHN_FIFO_#b)
>         static void evtchn_fifo_clear_pending(unsigned port)
>         {
>               event_word_t *word = event_word_from_port(port);
>               sync_clear_bit(EVTCHN_FIFO_BIT(PENDING, word), BM(word));
>         }
> similarly for the others.
> 
> If that is undesirable in the common code then wrap each existing helper
> in #ifndef evtchn_fifo_clean_pending and put something like the above in
> arch/arm64/include/asm/xen/events.h with a #define (and/or make the
> helpers themselves macros, or #define HAVE_XEN_FIFO_EVTCHN_ACCESSORS etc
> etc)
> 
> We still need your patch from
> http://article.gmane.org/gmane.comp.emulators.xen.devel/196067 for the
> other unaligned bitmap case. Note that this also removes the use of BM
> for non-PENDING/MASKED bits, which is why it was safe for me to change
> it above (also it would be safe to & ~0x7 after that change anyway).
> 
> Ian.
> 


Might be dealing with it on Xen side is only way unless arm64 folks say what
they think... because reports related to unaligned access still appear I'll
put the minimally tested patch for making arm64 bitops 32-bits friendly ;)

---
 arch/arm64/lib/bitops.S | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/lib/bitops.S b/arch/arm64/lib/bitops.S
index 7dac371..7c3b058 100644
--- a/arch/arm64/lib/bitops.S
+++ b/arch/arm64/lib/bitops.S
@@ -26,14 +26,14 @@
  */
        .macro  bitop, name, instr
 ENTRY( \name   )
-       and     w3, w0, #63             // Get bit offset
+       and     w3, w0, #31             // Get bit offset
        eor     w0, w0, w3              // Clear low bits
        mov     x2, #1
-       add     x1, x1, x0, lsr #3      // Get word offset
+       add     x1, x1, x0, lsr #2      // Get word offset
        lsl     x3, x2, x3              // Create mask
-1:     ldxr    x2, [x1]
-       \instr  x2, x2, x3
-       stxr    w0, x2, [x1]
+1:     ldxr    w2, [x1]
+       \instr  w2, w2, w3
+       stxr    w0, w2, [x1]
        cbnz    w0, 1b
        ret
 ENDPROC(\name  )
@@ -41,15 +41,15 @@ ENDPROC(\name       )
 
        .macro  testop, name, instr
 ENTRY( \name   )
-       and     w3, w0, #63             // Get bit offset
+       and     w3, w0, #31             // Get bit offset
        eor     w0, w0, w3              // Clear low bits
        mov     x2, #1
-       add     x1, x1, x0, lsr #3      // Get word offset
+       add     x1, x1, x0, lsr #2      // Get word offset
        lsl     x4, x2, x3              // Create mask
-1:     ldxr    x2, [x1]
+1:     ldxr    w2, [x1]
        lsr     x0, x2, x3              // Save old value of bit
-       \instr  x2, x2, x4              // toggle bit
-       stlxr   w5, x2, [x1]
+       \instr  w2, w2, w4              // toggle bit
+       stlxr   w5, w2, [x1]
        cbnz    w5, 1b
        dmb     ish
        and     x0, x0, #1
-- 
1.8.3


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