Re: [PATCH v2] RISC-V: Don't trust V from the riscv,isa DT property on T-Head CPUs

Jisheng Zhang
2023-07-13T17:24:15Z

On Thu, Jul 13, 2023 at 06:04:22PM +0100, Conor Dooley wrote:
> Jumping on top of Palmer's reply cos I had already started replying...
>
> On Thu, Jul 13, 2023 at 09:56:34AM -0700, Palmer Dabbelt wrote:
> > On Thu, 13 Jul 2023 09:36:49 PDT (-0700), jszhang@kernel.org wrote:
> > > On Wed, Jul 12, 2023 at 06:48:02PM +0100, Conor Dooley wrote:
> > > > From: Palmer Dabbelt <palmer@rivosinc.com>
> > > >
> > > > The last merge window contained both V support and the deprecation of
> > > > the riscv,isa DT property, with the V implementation reading riscv,isa
> > > > to determine the presence of the V extension. At the time that was the
> > > > only way to do it, but there's a lot of ambiguity around V in ISA
> > > > strings. In particular, there is a lot of firmware in the wild that
> > > > uses "v" in the riscv,isa DT property to communicate support for the
> > > > 0.7.1 version of the Vector specification implemented by T-Head CPU
> > > > cores.
> > >
> > > Add Guo
> > >
> > > Hi Conor, Palmer,
> > >
> > > FWICT, new T-HEAD's riscv cores such as C908 support standard RVV-1.0,
> > > this patch looks like a big hammer for T-HEAD. I do understand why
> >
> > Ya, it's a big hammer. There's no extant systems with the C908, though, and
> > given that the C906 and C910 alias marchid/mimplid it's kind of hard to
> > trust any of those values for T-Head systems. We could check for the 0s and
> > hope T-Head starts setting something else, but I'm not sure that's a net win
> > (we've also got the C920 in the Sophgo chip, which IIUC is V-0.7.1 too).
>
> (In reply to Jisheng mostly)
> It is most definitely a big hammer. And yes, we did talk about the c908
> & its standard implementation of vector before submitting this. Unless
> Guo can confirm that the c908 (and later CPU cores) will start setting
> mimpid & mvendorid, I don't really see what the alternatives are? *

In mainline kernel, three SoCs which powered by T-HEAD cpu are
supported: D1, D1s and TH1520, they don't contain the "v" in riscv,isa
dt property.

> Whacking in a list of DT compatibles to blacklist? That doesn't seem
> like something that would scale.
> Open to ideas on that front for sure, smaller hammers are always better!
>
> @Palmer, from what I am told, the c920 does put zeros in those CSRs,
> so we are okay on that front.
>
> * If they do do something other than 0s, the errata handling will need
> an update anyway, so the big hammer could be revised in tandem...
>
> > > this patch is provided, but can we mitigate the situation by carefully
> > > review the DTs? Per my understanding, dts is also part of linux kernel.
> >
> > That would break compatibility with existing firmware. It's certainly
> > something that has happened before, but we try to avoid it where possible.
>
> (Mostly in reply to Jisheng again)
> Sure, some devicetrees are part of the kernel, but not all are - they may
> be passed up from U-Boot or OpenSBI etc & contain "v" in riscv,isa.

If so this looks like a bug of u-boot and opensbi.

PS: does u-boot/opensbi modify "riscv,isa" property dynamically? Or
there's below usage case:
mainline linux kernel + dtb which is built from u-boot/opensbi source
code rather than linux kernel.

> The intent of this patch (and Palmer's v1) is to make sure such systems
> do not tell the kernel they support the standard version of v, when they
> do not do so.
>
> > In this case new T-Head systems that have standard V would just need to
> > provide the new DT properties instead of the ISA string property. We've
> > deprecated the ISA string property already so that's what new systems should
> > be doing anyway, and given that this only applies to new systems it doesn't
> > seem like a big ask.
>
> Aye, AFAIK there are no extant systems with a c908 outside of vendors?
> At least, from what searching I did I could not find a way to acquire
> one... If you read the patch, you will see that there is in fact a way
> out being provided & it is not a "no T-Head can never have vector"
> situation - just slightly earlier use of the new properties in the
> kernel than we perhaps intended.

This new properties look like a solution for T-HEAD vector support.
Guo may have comments.

Thank you.

>
> Thanks,
> Conor.
>
> > > > Rather than forcing use of the newly added interface that has strict
> > > > meanings for extensions to detect the presence of vector support, as
> > > > that would penalise those who have behaved, only ignore v in riscv,isa
> > > > on CPUs that report T-Head's vendor ID.
> > > >
> > > > Fixes: dc6667a4e7e3 ("riscv: Extending cpufeature.c to detect V-extension")
> > > > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> > > > Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
> > > > Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> > > > ---
> > > > Changes in v2:
> > > > - Use my version of the patch that touches hwcap and isainfo uniformly
> > > > - Don't penalise those who behaved
> > > > ---
> > > > arch/riscv/kernel/cpufeature.c | 22 ++++++++++++++++++++++
> > > > 1 file changed, 22 insertions(+)
> > > >
> > > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > > index bdcf460ea53d..05362715e1b7 100644
> > > > --- a/arch/riscv/kernel/cpufeature.c
> > > > +++ b/arch/riscv/kernel/cpufeature.c
> > > > @@ -21,6 +21,7 @@
> > > > #include <asm/hwcap.h>
> > > > #include <asm/patch.h>
> > > > #include <asm/processor.h>
> > > > +#include <asm/sbi.h>
> > > > #include <asm/vector.h>
> > > >
> > > > #define NUM_ALPHA_EXTS ('z' - 'a' + 1)
> > > > @@ -334,6 +335,27 @@ void __init riscv_fill_hwcap(void)
> > > > set_bit(RISCV_ISA_EXT_ZIHPM, isainfo->isa);
> > > > }
> > > >
> > > > + /*
> > > > + * "V" in ISA strings is ambiguous in practice: it should mean
> > > > + * just the standard V-1.0 but vendors aren't well behaved.
> > > > + * Many vendors with T-Head CPU cores which implement the 0.7.1
> > > > + * version of the vector specification put "v" into their DTs
> > > > + * and no T-Head CPU cores with the standard version of vector
> > > > + * are in circulation yet.
> > > > + * Platforms with T-Head CPU cores that support the standard
> > > > + * version of vector must provide the explicit V property,
> > > > + * which is well defined.
> > > > + */
> > > > + if (acpi_disabled && riscv_cached_mvendorid(cpu) == THEAD_VENDOR_ID) {
> > > > + if (of_property_match_string(node, "riscv,isa-extensions", "v") >= 0) {
> > > > + this_hwcap |= isa2hwcap[RISCV_ISA_EXT_v];
> > > > + set_bit(RISCV_ISA_EXT_v, isainfo->isa);
> > > > + } else {
> > > > + this_hwcap &= ~isa2hwcap[RISCV_ISA_EXT_v];
> > > > + clear_bit(RISCV_ISA_EXT_v, isainfo->isa);
> > > > + }
> > > > + }
> > > > +
> > > > /*
> > > > * All "okay" hart should have same isa. Set HWCAP based on
> > > > * common capabilities of every "okay" hart, in case they don't
> > > > --
> > > > 2.39.2
> > > >
> > > >
> > > > _______________________________________________
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> > > > linux-riscv@lists.infradead.org
> > > > http://lists.infradead.org/mailman/listinfo/linux-riscv