Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

[Guo Ren <guoren@kernel.org>]

On Tue, Mar 30, 2021 at 1:51 PM Anup Patel <anup@brainfault.org> wrote:
>
> On Tue, Mar 30, 2021 at 7:56 AM Guo Ren <guoren@kernel.org> wrote:
> >
> > On Mon, Mar 29, 2021 at 9:56 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > >
> > > On Mon, Mar 29, 2021 at 2:52 PM Guo Ren <guoren@kernel.org> wrote:
> > > >
> > > > On Mon, Mar 29, 2021 at 7:31 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > >
> > > > > On Mon, Mar 29, 2021 at 01:16:53PM +0200, Peter Zijlstra wrote:
> > > > > > Anyway, an additional 'funny' is that I suspect you cannot prove fwd
> > > > > > progress of the entire primitive with any of this on. But who cares
> > > > > > about details anyway.. :/
> > > > >
> > > > > What's the architectural guarantee on LL/SC progress for RISC-V ?
> > > >
> > > > funct5    | aq | rl   | rs2 |  rs1  | funct3 | rd | opcode
> > > >      5          1    1      5       5         3        5          7
> > > > LR.W/D  ordering  0     addr    width   dest    AMO
> > > > SC.W/D  ordering  src  addr    width   dest    AMO
> > > >
> > > > LR.W loads a word from the address in rs1, places the sign-extended
> > > > value in rd, and registers a reservation set—a set of bytes that
> > > > subsumes the bytes in the addressed word. SC.W conditionally writes a
> > > > word in rs2 to the address in rs1: the SC.W succeeds only if the
> > > > reservation is still valid and the reservation set contains the bytes
> > > > being written. If the SC.W succeeds, the instruction writes the word
> > > > in rs2 to memory, and it writes zero to rd. If the SC.W fails, the
> > > > instruction does not write to memory, and it writes a nonzero value to
> > > > rd. Regardless of success or failure, executing an SC.W instruction
> > > > *invalidates any reservation held by this hart*.
> > > >
> > > > More details, ref:
> > > > https://github.com/riscv/riscv-isa-manual
> > >
> > > I think section "3.5.3.2 Reservability PMA" [1] would be a more relevant
> > > link, as this defines memory areas that either do or do not have
> > > forward progress guarantees, including this part:
> > >
> > >    "When LR/SC is used for memory locations marked RsrvNonEventual,
> > >      software should provide alternative fall-back mechanisms used when
> > >      lack of progress is detected."
> > >
> > > My reading of this is that if the example you tried stalls, then either
> > > the PMA is not RsrvEventual, and it is wrong to rely on ll/sc on this,
> > > or that the PMA is marked RsrvEventual but the implementation is
> > > buggy.
> > Yes, PMA just defines physical memory region attributes, But in our
> > processor, when MMU is enabled (satp's value register > 2) in s-mode,
> > it will look at our custom PTE's attributes BIT(63) ref [1]:
> >
> >    PTE format:
> >    | 63 | 62 | 61 | 60 | 59 | 58-8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0
> >      SO   C    B    SH   SE    RSW   D   A   G   U   X   W   R   V
> >      ^    ^    ^    ^    ^
> >    BIT(63): SO - Strong Order
> >    BIT(62): C  - Cacheable
> >    BIT(61): B  - Bufferable
> >    BIT(60): SH - Shareable
> >    BIT(59): SE - Security
> >
> > So the memory also could be RsrvNone/RsrvEventual.
> >
> > [1] https://github.com/c-sky/csky-linux/commit/e837aad23148542771794d8a2fcc52afd0fcbf88
>
> Is this about your C-sky architecture or your RISC-V implementation.
It's in RISC-V implementation.

>
> If these PTE bits are in your RISC-V implementation then clearly your
> RISC-V implementation is not compliant with the RISC-V privilege spec
> because these bits are not defined in RISC-V privilege spec.
We could disable it if the vendor's SOC has a coherency interconnect
bus, so C910 is compliant with standard privilege spec.

ps:
I remember someone has mentioned a similar design in 1.12-draft-VM-TASKGROUP:

"Bit 63 indicates that the PTE uses a custom implementation-specific
encoding. If bit 63 is set, the algorithm for virtual-to-physical
address translation is implementation-defined. If bit 63 is not set,
the algorithm for virtual-to-physical address translation is described
in Section 4.4.2.

Bit 62 indicates the use of naturally aligned power-of-two (NAPOT)
address translation contiguity, as described in Section 4.4.2.

Bits 61–xx indicate cacheability attributes associated with the
virtual address in question, as de-scribed in Section 4.4.3.

Bits xx–54 are reserved for future use."


>
> Regards,
> Anup
> >
> > >
> > > It also seems that the current "amoswap" based implementation
> > > would be reliable independent of RsrvEventual/RsrvNonEventual.
> > Yes, the hardware implementation of AMO could be different from LR/SC.
> > AMO could use ACE snoop holding to lock the bus in hw coherency
> > design, but LR/SC uses an exclusive monitor without locking the bus.
> >
> > > arm64 is already in the situation of having to choose between
> > > two cmpxchg() implementation at runtime to allow falling back to
> > > a slower but more general version, but it's best to avoid that if you
> > > can.
> > Current RISC-V needn't multiple versions to select, and all AMO &
> > LR/SC has been defined in the spec.
> >
> > RISC-V hasn't CAS instructions, and it uses LR/SC for cmpxchg. I don't
> > think LR/SC would be slower than CAS, and CAS is just good for code
> > size.
> >
> > >
> > >          Arnd
> > >
> > > [1] http://www.five-embeddev.com/riscv-isa-manual/latest/machine.html#atomicity-pmas
> >
> > --
> > Best Regards
> >  Guo Ren
> >
> > ML: https://lore.kernel.org/linux-csky/



--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/