Re: [cfe-users] RFC v2: Re: cacheflush.2
Hi Michael, On 12/11/20 9:15 AM, Michael Kerrisk (man-pages) wrote: > i Alex, > > On 12/10/20 9:56 PM, Alejandro Colomar (man-pages) wrote: >> Hi all, >> >> v2: >> >> [ >> NOTES >>Unless you need the finer grained control that this system >>call provides, you probably want to use the GCC built-in >>function __builtin___clear_cache(), which provides a more >>portable interface: >> >>void __builtin___clear_cache(void *begin, void *end); >> ] > > This seems a reasonable text to me, but I think it would be helpful > to say a little more precisely what kind of portability we are > talking about here. Sure. > > Greater ortability across Linux architectures? Greater portability > across platforms supported by GCC (including non-Linux) platforms? > Something else? '... which provides a portable interface across platforms supported by GCC:' sounds good. Maybe GCC devs have something more to add. Thanks, Alex > > Thanks, > > Michael > > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
[cfe-users] RFC v2: Re: cacheflush.2
Hi all, v2: [ NOTES Unless you need the finer grained control that this system call provides, you probably want to use the GCC built-in function __builtin___clear_cache(), which provides a more portable interface: void __builtin___clear_cache(void *begin, void *end); ] If you like it, I'll send the patch. BTW, I'll also have a look and document the different prototypes for cacheflush(2). Thanks, Alex On 12/10/20 8:20 PM, Heinrich Schuchardt wrote: > On 12/10/20 7:17 PM, Dave Martin wrote: >> On Wed, Dec 09, 2020 at 07:34:09PM +0100, Alejandro Colomar >> (man-pages) wrote: >>> Hi Heinrich & Michael, >>> >>> What about the following?: >>> >>> [ >>> NOTES >>> GCC provides a similar function, which may be useful on archi‐ >>> tectures that lack this system call: >>> >>> void __builtin___clear_cache(void *begin, void *end); >>> ] >>> >>> Cheers, >>> >>> Alex >> >> Maybe we should discourage people from calling the cacheflush syscall? >> >> I think that people shouldn't be using the syscall unless they really >> need the finer grained control it provides, and are prepared to take a >> hit to portability. >> >> (On arches where userspace is allowed to do cache flushing directly, >> __builtin___clear_cache() should transparently do the right thing, with >> no syscall overhead -- if not, that's probably a bug in the toolchain or >> compiler support library.) > > What the compiler builtin does depends on the architecture (e.g. nothing > for x86, cacheflush() for MIPS, a private syscall (0xf0002) on ARM, > assembly code on ARM64, ...) and on the the operating system (Linux, > BSD, OS X). For portable code the builtin is really the best choice. > > Best regards > > Heinrich > >> >> [...] >> >> Cheers >> ---Dave >> > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] cacheflush.2
Hi Heinrich, It looks like a bug (or at least an undocumented divergence from GCC) in Clang/LLVM. Or I couldn't find the documentation for it. Clang uses 'char *': https://github.com/llvm/llvm-project/blob/7faf62a80bfc3a9dfe34133681fcc31f8e8d658b/clang/include/clang/Basic/Builtins.def#L583 GCC uses 'void *': https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html I CCd Clang and GCC lists; maybe they know about that divergence. Cheers, Alex On 12/9/20 7:48 PM, Heinrich Schuchardt wrote: > On 12/9/20 7:34 PM, Alejandro Colomar (man-pages) wrote: >> Hi Heinrich & Michael, >> >> What about the following?: >> >> [ >> NOTES >> GCC provides a similar function, which may be useful on archi‐ >> tectures that lack this system call: >> >> void __builtin___clear_cache(void *begin, void *end); >> ] > > I just checked building with Clang/LLVM. There the arguments are of type > (char *). See the following error output: > > +arch/sandbox/cpu/cache.c:19:26: error: passing 'uint8_t *' (aka > 'unsigned char *') to parameter of type 'char *' converts between > pointers to integer types with different sign [-Werror,-Wpointer-sign] > + __builtin___clear_cache(state->ram_buf, > + ^~ > +arch/sandbox/cpu/cache.c:20:12: error: passing 'uint8_t *' (aka > 'unsigned char *') to parameter of type 'char *' converts between > pointers to integer types with different sign [-Werror,-Wpointer-sign] > + state->ram_buf + state->ram_size); > + ^~~~ > > Best regards > > Heinrich > >> >> Cheers, >> >> Alex >> >> On 12/9/20 7:04 PM, Heinrich Schuchardt wrote: >>> Hello Michael, >>> >>> function cacheflush() does not exist on many architectures. >>> >>> It would have saved me a lot of time if the man-page had referenced >>> GCC's >>> >>> void __builtin___clear_cache(void *begin, void *end) >>> >>> Maybe you can add it to NOTES. >>> >>> Best regards >>> >>> heirnich >> > -- Alejandro Colomar Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] cacheflush.2
It looks like GCC recently moved from 'char *' to 'void *'. This SO question[1] (4 years ago) quotes the GCC docs and they had 'char *'. Maybe Clang hasn't noticed the change. I'll report a bug. [1]: https://stackoverflow.com/q/35741814/6872717 On 12/9/20 8:15 PM, Alejandro Colomar (man-pages) wrote: > Hi Heinrich, > > It looks like a bug (or at least an undocumented divergence from GCC) in > Clang/LLVM. Or I couldn't find the documentation for it. > > Clang uses 'char *': > https://github.com/llvm/llvm-project/blob/7faf62a80bfc3a9dfe34133681fcc31f8e8d658b/clang/include/clang/Basic/Builtins.def#L583 > > GCC uses 'void *': > https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html > > I CCd Clang and GCC lists; maybe they know about that divergence. > > Cheers, > > Alex > > On 12/9/20 7:48 PM, Heinrich Schuchardt wrote: >> On 12/9/20 7:34 PM, Alejandro Colomar (man-pages) wrote: >>> Hi Heinrich & Michael, >>> >>> What about the following?: >>> >>> [ >>> NOTES >>> GCC provides a similar function, which may be useful on archi‐ >>> tectures that lack this system call: >>> >>> void __builtin___clear_cache(void *begin, void *end); >>> ] >> >> I just checked building with Clang/LLVM. There the arguments are of type >> (char *). See the following error output: >> >> +arch/sandbox/cpu/cache.c:19:26: error: passing 'uint8_t *' (aka >> 'unsigned char *') to parameter of type 'char *' converts between >> pointers to integer types with different sign [-Werror,-Wpointer-sign] >> + __builtin___clear_cache(state->ram_buf, >> + ^~ >> +arch/sandbox/cpu/cache.c:20:12: error: passing 'uint8_t *' (aka >> 'unsigned char *') to parameter of type 'char *' converts between >> pointers to integer types with different sign [-Werror,-Wpointer-sign] >> + state->ram_buf + state->ram_size); >> + ^~~~ >> >> Best regards >> >> Heinrich >> >>> >>> Cheers, >>> >>> Alex >>> >>> On 12/9/20 7:04 PM, Heinrich Schuchardt wrote: Hello Michael, function cacheflush() does not exist on many architectures. It would have saved me a lot of time if the man-page had referenced GCC's void __builtin___clear_cache(void *begin, void *end) Maybe you can add it to NOTES. Best regards heirnich >>> >> > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
[cfe-users] RFC v3: Re: cacheflush.2
Hi all, Please review this text: [ NOTES Unless you need the finer grained control that this system call provides, you probably want to use the GCC built-in function __builtin___clear_cache(), which provides a more portable interface: void __builtin___clear_cache(void *begin, void *end); On platforms that don't require instruction cache flushes, __builtin___clear_cache() has no effect. Note: On some GCC-compatible compilers, such as clang, the prototype for this function uses char * instead of void *. ] Thanks, Alex On 12/11/20 7:02 PM, Alejandro Colomar (man-pages) wrote: > Hi Michael, > > On 12/11/20 9:15 AM, Michael Kerrisk (man-pages) wrote: >> i Alex, >> >> On 12/10/20 9:56 PM, Alejandro Colomar (man-pages) wrote: >>> Hi all, >>> >>> v2: >>> >>> [ >>> NOTES >>>Unless you need the finer grained control that this system >>>call provides, you probably want to use the GCC built-in >>>function __builtin___clear_cache(), which provides a more >>>portable interface: >>> >>>void __builtin___clear_cache(void *begin, void *end); >>> ] >> >> This seems a reasonable text to me, but I think it would be helpful >> to say a little more precisely what kind of portability we are >> talking about here. > Sure. > >> >> Greater ortability across Linux architectures? Greater portability >> across platforms supported by GCC (including non-Linux) platforms? >> Something else? > > '... which provides a portable interface across platforms supported by > GCC:' sounds good. > > Maybe GCC devs have something more to add. > > Thanks, > > Alex > >> >> Thanks, >> >> Michael >> >> > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
[cfe-users] RFC v4: Re: cacheflush.2
I forgot to add a junk to the text. v4: NOTES Unless you need the finer grained control that this system call provides, you probably want to use the GCC built-in function __builtin___clear_cache(), which provides a portable interface across platforms supported by GCC and compatible compilers: //Maybe 'and compatible compilers' is redundant and I should remove it? void __builtin___clear_cache(void *begin, void *end); On platforms that don't require instruction cache flushes, __builtin___clear_cache() has no effect. Note: On some GCC-compatible compilers, such as clang, the prototype for this function uses char * instead of void *. On 12/11/20 7:22 PM, Alejandro Colomar (man-pages) wrote: > Hi all, > > Please review this text: > > [ > NOTES >Unless you need the finer grained control that this system >call provides, you probably want to use the GCC built-in >function __builtin___clear_cache(), which provides a more >portable interface: > >void __builtin___clear_cache(void *begin, void *end); > >On platforms that don't require instruction cache flushes, >__builtin___clear_cache() has no effect. > >Note: On some GCC-compatible compilers, such as clang, the >prototype for this function uses char * instead of void *. > ] > > Thanks, > > Alex > > On 12/11/20 7:02 PM, Alejandro Colomar (man-pages) wrote: >> Hi Michael, >> >> On 12/11/20 9:15 AM, Michael Kerrisk (man-pages) wrote: >>> i Alex, >>> >>> On 12/10/20 9:56 PM, Alejandro Colomar (man-pages) wrote: Hi all, v2: [ NOTES Unless you need the finer grained control that this system call provides, you probably want to use the GCC built-in function __builtin___clear_cache(), which provides a more portable interface: void __builtin___clear_cache(void *begin, void *end); ] >>> >>> This seems a reasonable text to me, but I think it would be helpful >>> to say a little more precisely what kind of portability we are >>> talking about here. >> Sure. >> >>> >>> Greater ortability across Linux architectures? Greater portability >>> across platforms supported by GCC (including non-Linux) platforms? >>> Something else? >> >> '... which provides a portable interface across platforms supported by >> GCC:' sounds good. >> >> Maybe GCC devs have something more to add. > >> >> Thanks, >> >> Alex >> >>> >>> Thanks, >>> >>> Michael >>> >>> >> > -- Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] RFC v2: Re: cacheflush.2
i Alex, On 12/10/20 9:56 PM, Alejandro Colomar (man-pages) wrote: > Hi all, > > v2: > > [ > NOTES >Unless you need the finer grained control that this system >call provides, you probably want to use the GCC built-in >function __builtin___clear_cache(), which provides a more >portable interface: > >void __builtin___clear_cache(void *begin, void *end); > ] This seems a reasonable text to me, but I think it would be helpful to say a little more precisely what kind of portability we are talking about here. Greater ortability across Linux architectures? Greater portability across platforms supported by GCC (including non-Linux) platforms? Something else? Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
