Re: [PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

[Yury Norov]

On Wed, Dec 02, 2015 at 09:37:05AM +0100, Arnd Bergmann wrote:
> On Wednesday 02 December 2015 02:35:03 Yury Norov wrote:
> > On Tue, Dec 01, 2015 at 11:39:42PM +0100, Arnd Bergmann wrote:
> > > On Wednesday 02 December 2015 00:29:04 Yury Norov wrote:
> > > I'm not sure I understand this part. What changes specifically do we need?
> > > 
> > 
> > For kernel:
> > 
> >         diff --git a/arch/arm64/include/asm/shmparam.h b/arch/arm64/include/asm/shmparam.h
> >         index 4df608a..e368a55 100644
> >         --- a/arch/arm64/include/asm/shmparam.h
> >         +++ b/arch/arm64/include/asm/shmparam.h
> >         @@ -21,7 +21,7 @@
> >           * alignment value. Since we don't have aliasing D-caches, the rest of
> >           * the time we can safely use PAGE_SIZE.
> >           */
> >         -#define COMPAT_SHMLBA	0x4000
> >         +#define COMPAT_SHMLBA	(4 * PAGE_SIZE)
> >          
> >          #include <asm-generic/shmparam.h>
> >          
> >         diff --git a/arch/arm64/kernel/sys_ilp32.c b/arch/arm64/kernel/sys_ilp32.c
> >         index c5bc712..877bedf 100644
> >         --- a/arch/arm64/kernel/sys_ilp32.c
> >         +++ b/arch/arm64/kernel/sys_ilp32.c
> >         @@ -42,15 +42,12 @@ asmlinkage long sys_rt_sigreturn_wrapper(void);
> >          #define compat_sys_pwrite64            sys_pwrite64
> >          #define compat_sys_readahead           sys_readahead
> >          #define compat_sys_rt_sigaction        sys_rt_sigaction
> >         -#define compat_sys_shmat               sys_shmat
> >          #define compat_sys_sync_file_range     sys_sync_file_range
> >          #define compat_sys_truncate64          sys_truncate
> >          #define compat_sys_sigaltstack         sys_sigaltstack
> >  
> > For library - just create a header in ilp32 directory that defines
> > SHMLBA exactly as arm: __getpagesize () << 2
> 
> I think we both reversed our positions here ;-)
> 
> The 4*PAGE_SIZE on ARM is an architecture specific oddity, I believe
> to work around aliasing caches on ARMv6. As no other architecture does
> this, we're probably better off not duplicating it for aarch64-ilp32
> and just use sys_shmat as your v6 patch does.
> 
> > > It sounds like shmat is one of the cases we an override makes sense
> > > and we should use sys_shmat with PAGE_SIZE for aarch64 ilp32 mode.
> > > 
> > 
> > [...]
> > 
> > > We normally assume that the page size on ARM is fixed to 4K, so
> > > there might be user space that just hardcodes 16K SHMLBA 
> > 
> > It means that we should use compat_sys_shmat because hardcoded
> > userspace may have a chance to work for 4K pages. If we'll use
> > sys_shmat (and so 4K SHMLBA), we'll definitely make hardcoded
> > userspace broken. Non-hardcoded userspace will work anyway.
> > 
> > We can describe in documentation that 4k pages are prefferable.
> 
> The hardcoded user space would only apply to old source code that
> specifically tries to adapt to what ARM does, but gets it wrong,
> If we have source code that is converted from x86 or mips and
> hardcodes anything, it's more likely to be the normal __getpagesize().
> 
> 	Arnd

Hmm... OK. Let's have non-compat shmat here...
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