Re: [PATCH V11 00/17] riscv: Add Native/Paravirt qspinlock support

[Guo Ren <guoren@kernel.org>]

On Mon, Sep 11, 2023 at 3:45 AM Conor Dooley <conor@kernel.org> wrote:
>
> On Sun, Sep 10, 2023 at 05:49:13PM +0800, Guo Ren wrote:
> > On Sun, Sep 10, 2023 at 5:32 PM Conor Dooley <conor@kernel.org> wrote:
> > >
> > > On Sun, Sep 10, 2023 at 05:16:46PM +0800, Guo Ren wrote:
> > > > On Sun, Sep 10, 2023 at 4:58 PM Conor Dooley <conor@kernel.org> wrote:
> > > > >
> > > > > On Sun, Sep 10, 2023 at 04:28:54AM -0400, guoren@kernel.org wrote:
> > > > >
> > > > > > Changlog:
> > > > > > V11:
> > > > > >  - Based on Leonardo Bras's cmpxchg_small patches v5.
> > > > > >  - Based on Guo Ren's Optimize arch_spin_value_unlocked patch v3.
> > > > > >  - Remove abusing alternative framework and use jump_label instead.
> > > > >
> > > > > btw, I didn't say that using alternatives was the problem, it was
> > > > > abusing the errata framework to perform feature detection that I had
> > > > > a problem with. That's not changed in v11.
> > > > I've removed errata feature detection. The only related patches are:
> > > >  - riscv: qspinlock: errata: Add ERRATA_THEAD_WRITE_ONCE fixup
> > > >  - riscv: qspinlock: errata: Enable qspinlock for T-HEAD processors
> > > >
> > > > Which one is your concern? Could you reply on the exact patch thread? Thx.
> > >
> > > riscv: qspinlock: errata: Enable qspinlock for T-HEAD processors
> > >
> > > Please go back and re-read the comments I left on v11 about using the
> > > errata code for feature detection.
> > >
> > > > > A stronger forward progress guarantee is not an erratum, AFAICT.
> > >
> > > > Sorry, there is no erratum of "stronger forward progress guarantee" in the V11.
> > >
> > > "riscv: qspinlock: errata: Enable qspinlock for T-HEAD processors" still
> > > uses the errata framework to detect the presence of the stronger forward
> > > progress guarantee in v11.
> > Oh, thx for pointing it out. I could replace it with this:
> >
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index 88690751f2ee..4be92766d3e3 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -310,7 +310,8 @@ static void __init riscv_spinlock_init(void)
> >  {
> >  #ifdef CONFIG_RISCV_COMBO_SPINLOCKS
> >         if (!enable_qspinlock_key &&
> > -           (sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM)) {
> > +           (sbi_get_firmware_id() != SBI_EXT_BASE_IMPL_ID_KVM) &&
> > +           (sbi_get_mvendorid() != THEAD_VENDOR_ID)) {
> >                 static_branch_disable(&combo_qspinlock_key);
> >                 pr_info("Ticket spinlock: enabled\n");
> >         } else {
>
> As I said on v11, I am opposed to feature probing using mvendorid & Co,
> partially due to the exact sort of check here to see if the kernel is
> running as a KVM guest. IMO, whether a platform has this stronger
KVM can't use any fairness lock, so forcing it using a Test-Set lock
or paravirt qspinlock is the right way. KVM is not a vendor platform.

> guarantee needs to be communicated by firmware, using ACPI or DT.
> I made some comments on v11, referring similar discussion about the
> thead vector stuff. Please go take a look at that.
I prefer forcing T-HEAD processors using qspinlock, but if all people
thought it must be in the ACPI or DT, I would compromise. Then, I
would delete the qspinlock cmdline param patch and move it into DT.

By the way, what's the kind of DT format? How about:
        cpus {
                #address-cells = <1>;
                #size-cells = <0>;
+              qspinlock;
                cpu0: cpu@0 {
                        compatible = "sifive,bullet0", "riscv";
                        device_type = "cpu";
                        i-cache-block-size = <64>;
                        i-cache-sets = <128>;

--
Best Regards
 Guo Ren