Ian Campbell
2013-Apr-24 13:21 UTC
[PATCH] xen: arm: let the compiler choose the dummy register for TLB flush asm
While staring at some disassembly I noticed that the compiler was jumping
through hoops to preserve the function argument (in r0) simply so it could use
r0 as the dummy argument to the tlb flush cp register writes.
Remove the enforced use of r0 and use an output constraint for the dummy
register so there is no need to initialise the dummy.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
---
xen/include/asm-arm/arm32/page.h | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/xen/include/asm-arm/arm32/page.h b/xen/include/asm-arm/arm32/page.h
index d295316..86940ba 100644
--- a/xen/include/asm-arm/arm32/page.h
+++ b/xen/include/asm-arm/arm32/page.h
@@ -35,7 +35,7 @@ static inline void write_pte(lpae_t *p, lpae_t pte)
*/
static inline void flush_xen_text_tlb(void)
{
- register unsigned long r0 asm ("r0");
+ unsigned long dummy;
asm volatile (
"isb;" /* Ensure synchronization with
previous changes to text */
STORE_CP32(0, TLBIALLH) /* Flush hypervisor TLB */
@@ -43,7 +43,7 @@ static inline void flush_xen_text_tlb(void)
STORE_CP32(0, BPIALL) /* Flush branch predictor */
"dsb;" /* Ensure completion of TLB+BP
flush */
"isb;"
- : : "r" (r0) /*dummy*/ : "memory");
+ : "=r" (dummy) : : "memory");
}
/*
@@ -52,12 +52,12 @@ static inline void flush_xen_text_tlb(void)
*/
static inline void flush_xen_data_tlb(void)
{
- register unsigned long r0 asm ("r0");
+ unsigned long dummy;
asm volatile("dsb;" /* Ensure preceding are visible */
STORE_CP32(0, TLBIALLH)
"dsb;" /* Ensure completion of the TLB flush */
"isb;"
- : : "r" (r0) /* dummy */: "memory");
+ : "=r" (dummy) : : "memory");
}
/*
--
1.7.2.5
Tim Deegan
2013-Apr-24 14:17 UTC
Re: [PATCH] xen: arm: let the compiler choose the dummy register for TLB flush asm
At 14:21 +0100 on 24 Apr (1366813285), Ian Campbell wrote:> While staring at some disassembly I noticed that the compiler was jumping > through hoops to preserve the function argument (in r0) simply so it could use > r0 as the dummy argument to the tlb flush cp register writes. > > Remove the enforced use of r0 and use an output constraint for the dummy > register so there is no need to initialise the dummy.Since we don''t care about the register''s contents and it''s not changed, can we just drop the variable and constraint entirely? Tim.> Signed-off-by: Ian Campbell <ian.campbell@citrix.com> > --- > xen/include/asm-arm/arm32/page.h | 8 ++++---- > 1 files changed, 4 insertions(+), 4 deletions(-) > > diff --git a/xen/include/asm-arm/arm32/page.h b/xen/include/asm-arm/arm32/page.h > index d295316..86940ba 100644 > --- a/xen/include/asm-arm/arm32/page.h > +++ b/xen/include/asm-arm/arm32/page.h > @@ -35,7 +35,7 @@ static inline void write_pte(lpae_t *p, lpae_t pte) > */ > static inline void flush_xen_text_tlb(void) > { > - register unsigned long r0 asm ("r0"); > + unsigned long dummy; > asm volatile ( > "isb;" /* Ensure synchronization with previous changes to text */ > STORE_CP32(0, TLBIALLH) /* Flush hypervisor TLB */ > @@ -43,7 +43,7 @@ static inline void flush_xen_text_tlb(void) > STORE_CP32(0, BPIALL) /* Flush branch predictor */ > "dsb;" /* Ensure completion of TLB+BP flush */ > "isb;" > - : : "r" (r0) /*dummy*/ : "memory"); > + : "=r" (dummy) : : "memory"); > } > > /* > @@ -52,12 +52,12 @@ static inline void flush_xen_text_tlb(void) > */ > static inline void flush_xen_data_tlb(void) > { > - register unsigned long r0 asm ("r0"); > + unsigned long dummy; > asm volatile("dsb;" /* Ensure preceding are visible */ > STORE_CP32(0, TLBIALLH) > "dsb;" /* Ensure completion of the TLB flush */ > "isb;" > - : : "r" (r0) /* dummy */: "memory"); > + : "=r" (dummy) : : "memory"); > } > > /* > -- > 1.7.2.5 >
Ian Campbell
2013-Apr-24 15:14 UTC
Re: [PATCH] xen: arm: let the compiler choose the dummy register for TLB flush asm
On Wed, 2013-04-24 at 15:17 +0100, Tim Deegan wrote:> At 14:21 +0100 on 24 Apr (1366813285), Ian Campbell wrote: > > While staring at some disassembly I noticed that the compiler was jumping > > through hoops to preserve the function argument (in r0) simply so it could use > > r0 as the dummy argument to the tlb flush cp register writes. > > > > Remove the enforced use of r0 and use an output constraint for the dummy > > register so there is no need to initialise the dummy. > > Since we don''t care about the register''s contents and it''s not changed, > can we just drop the variable and constraint entirely?and just hard code the r0? I hadn''t though of that, I suppose we could yes. Ian.
Ian Campbell
2013-Apr-25 15:20 UTC
Re: [PATCH] xen: arm: let the compiler choose the dummy register for TLB flush asm
On Wed, 2013-04-24 at 16:14 +0100, Ian Campbell wrote:> On Wed, 2013-04-24 at 15:17 +0100, Tim Deegan wrote: > > At 14:21 +0100 on 24 Apr (1366813285), Ian Campbell wrote: > > > While staring at some disassembly I noticed that the compiler was jumping > > > through hoops to preserve the function argument (in r0) simply so it could use > > > r0 as the dummy argument to the tlb flush cp register writes. > > > > > > Remove the enforced use of r0 and use an output constraint for the dummy > > > register so there is no need to initialise the dummy. > > > > Since we don''t care about the register''s contents and it''s not changed, > > can we just drop the variable and constraint entirely? > > and just hard code the r0? I hadn''t though of that, I suppose we could > yes.Scuppered because our clever STORE_CP32 macros add % to their argument. I don''t really want to change those macros, I suppose I could add DUMMY_CP32 which just takes a name and inputs a dummy reigster. Ian