RISCV Vector unit disabled by default for new task (was Re: [PATCH v12 17/17] riscv: prctl to enable vector commands)

[Andrew Pinski]

On Thu, Dec 15, 2022 at 10:59 AM Andrew Pinski <pinskia at gmail.com> wrote:
>
> On Thu, Dec 15, 2022 at 10:57 AM Vineet Gupta <vineetg at rivosinc.com> wrote:
> >
> >
> >
> > On 12/15/22 07:33, Richard Henderson wrote:
> > > On 12/15/22 04:28, Florian Weimer via Libc-alpha wrote:
> > >> * Björn Töpel:
> > >>
> > >>>> For SVE, it is in fact disabled by default in the kernel.  When a
> > >>>> thread
> > >>>> executes the first SVE instruction, it will cause an exception, the
> > >>>> kernel
> > >>>> will allocate memory for SVE state and enable TIF_SVE. Further use
> > >>>> of SVE
> > >>>> instructions will proceed without exceptions.  Although SVE is
> > >>>> disabled by
> > >>>> default, it is enabled automatically.  Since this is done
> > >>>> automatically
> > >>>> during an exception handler, there is no opportunity for memory
> > >>>> allocation
> > >>>> errors to be reported, as there are in the AMX case.
> > >>>
> > >>> Glibc has an SVE optimized memcpy, right? Doesn't that mean that pretty
> > >>> much all processes on an SVE capable system will enable SVE
> > >>> (lazily)? If
> > >>> so, that's close to "enabled by default" (unless SVE is disabled system
> > >>> wide).
> > >>
> > >> Yes, see sysdeps/aarch64/multiarch/memcpy.c:
> > >>
> > >>    static inline __typeof (__redirect_memcpy) *
> > >>    select_memcpy_ifunc (void)
> > >>    {
> > >>      INIT_ARCH ();
> > >>         if (sve && HAVE_AARCH64_SVE_ASM)
> > >>        {
> > >>          if (IS_A64FX (midr))
> > >>            return __memcpy_a64fx;
> > >>          return __memcpy_sve;
> > >>        }
> > >>         if (IS_THUNDERX (midr))
> > >>        return __memcpy_thunderx;
> > >>         if (IS_THUNDERX2 (midr) || IS_THUNDERX2PA (midr))
> > >>        return __memcpy_thunderx2;
> > >>         if (IS_FALKOR (midr) || IS_PHECDA (midr))
> > >>        return __memcpy_falkor;
> > >>         return __memcpy_generic;
> > >>    }
> > >>    And the __memcpy_sve implementation actually uses SVE.
> > >>
> > >> If there were a prctl to select the vector width and enable the vector
> > >> extension, we'd have to pick a width in glibc anyway.
> > >
> > > There *is* a prctl to adjust the SVE vector width, but glibc does not
> > > need to select because SVE dynamically adjusts to the currently
> > > enabled width.  The kernel selects a default width that fits within
> > > the default signal frame size.
> > >
> > > The other thing of note for SVE is that, with the default function ABI
> > > all of the SVE state is call-clobbered, which allows the kernel to
> > > drop instead of save state across system calls.  (There is a separate
> > > vector function call ABI when SVE types are used.)
> >
> > For the RV psABI, it is similar - all V regs are
> > caller-saved/call-clobbered [1] and syscalls are not required to
> > preserve V regs [2]
> > However last I checked ARM documentation the ABI doc seemed to suggest
> > that some (parts) of the SVE regs are callee-saved [3]
>
> Yes the lower 64 bits which overlap with the floating point registers.

I should expand on that, only the specific callee registers have to
save the lower 64bits because they overlap with the floating point
registers.

Thanks,
Andrew

>
> Thanks,
> Andrew Pinski
>
>
> >
> > >
> > > So while strcpy may enable SVE for the thread, the next syscall may
> > > disable it again.
> >
> > Next syscall could trash them, but will it disable SVE ? Despite
> > syscall/function-call clobbers, using V in tight loops such as mem*/str*
> > still is a win.
> >
> >
> > [1]
> > https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc
> > [2]
> > https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc
> > [3]
> > https://github.com/ARM-software/abi-aa/blob/2982a9f3b512a5bfdc9e3fea5d3b298f9165c36b/aapcs64/aapcs64.rst#the-base-procedure-call-standard
> > Sec 6.1.3 ".... In other cases it need only preserve the low 64 bits of
> > z8-z15"
> >