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Date:   Mon, 10 Jul 2023 09:59:40 +0200
From:   Samuel Ortiz <sameo@rivosinc.com>
To:     Stefan O'Rear <sorear@fastmail.com>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux@rivosinc.com,
        Conor Dooley <conor.dooley@microchip.com>,
        Andrew Jones <ajones@ventanamicro.com>,
        Heiko Stuebner <heiko.stuebner@vrull.eu>,
        Anup Patel <apatel@ventanamicro.com>,
        linux-kernel@vger.kernel.org,
        "Hongren (Zenithal) Zheng" <i@zenithal.me>,
        Guo Ren <guoren@kernel.org>, Atish Patra <atishp@rivosinc.com>,
        =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= <bjorn@rivosinc.com>,
        Evan Green <evan@rivosinc.com>
Subject: Re: [PATCH 2/3] RISC-V: hwprobe: Expose Zbc and the scalar crypto
 extensions
Message-ID: <ZKu6bCMPX0WKetVp@vermeer>
References: <20230627143747.1599218-1-sameo@rivosinc.com>
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On Wed, Jun 28, 2023 at 09:25:02AM -0400, Stefan O'Rear wrote:
> On Wed, Jun 28, 2023, at 6:04 AM, Samuel Ortiz wrote:
> > On Tue, Jun 27, 2023 at 08:34:20PM -0400, Stefan O'Rear wrote:
> >> On Tue, Jun 27, 2023, at 10:37 AM, Samuel Ortiz wrote:
> >> > Zbc was missing from a previous Bit-Manipulation extension hwprobe
> >> > patch.
> >> >
> >> > Add all scalar crypto extensions bits, and define a macro for setting
> >> > the hwprobe key/pair in a more readable way.
> >> >
> >> > Signed-off-by: Samuel Ortiz <sameo@rivosinc.com>
> >> > ---
> >> >  Documentation/riscv/hwprobe.rst       | 33 ++++++++++++++++++++++++
> >> >  arch/riscv/include/uapi/asm/hwprobe.h | 11 ++++++++
> >> >  arch/riscv/kernel/sys_riscv.c         | 36 ++++++++++++++++-----------
> >> >  3 files changed, 66 insertions(+), 14 deletions(-)
> >> >
> >> > diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
> >> > index 19165ebd82ba..3177550106e0 100644
> >> > --- a/Documentation/riscv/hwprobe.rst
> >> > +++ b/Documentation/riscv/hwprobe.rst
> >> > @@ -72,11 +72,44 @@ The following keys are defined:
> >> >         extensions.
> >> > 
> >> >    * :c:macro:`RISCV_HWPROBE_EXT_ZBB`: The Zbb extension is supported, 
> >> > as defined
> >> > +      in version 1.0 of the Bit-Manipulation ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZBC`: The Zbc extension is supported, 
> >> > as defined
> >> >         in version 1.0 of the Bit-Manipulation ISA extensions.
> >> > 
> >> >    * :c:macro:`RISCV_HWPROBE_EXT_ZBS`: The Zbs extension is supported, 
> >> > as defined
> >> >         in version 1.0 of the Bit-Manipulation ISA extensions.
> >> > 
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZBKB`: The Zbkb extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZBKC`: The Zbkc extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZBKX`: The Zbkx extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKND`: The Zknd extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKNE`: The Zkne extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKNH`: The Zknh extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKR`: The Zkr extension is supported, 
> >> > as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKSED`: The Zksed extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKSH`: The Zksh extension is 
> >> > supported, as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> > +  * :c:macro:`RISCV_HWPROBE_EXT_ZKT`: The Zkt extension is supported, 
> >> > as defined
> >> > +    in version 1.0 of the Scalar Cryptography ISA extensions.
> >> > +
> >> >  * :c:macro:`RISCV_HWPROBE_KEY_CPUPERF_0`: A bitmask that contains 
> >> > performance
> >> >    information about the selected set of processors.
> >> > 
> >> > diff --git a/arch/riscv/include/uapi/asm/hwprobe.h 
> >> > b/arch/riscv/include/uapi/asm/hwprobe.h
> >> > index 006bfb48343d..8357052061b3 100644
> >> > --- a/arch/riscv/include/uapi/asm/hwprobe.h
> >> > +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> >> > @@ -29,6 +29,17 @@ struct riscv_hwprobe {
> >> >  #define		RISCV_HWPROBE_EXT_ZBA		(1 << 3)
> >> >  #define		RISCV_HWPROBE_EXT_ZBB		(1 << 4)
> >> >  #define		RISCV_HWPROBE_EXT_ZBS		(1 << 5)
> >> > +#define		RISCV_HWPROBE_EXT_ZBC		(1 << 6)
> >> > +#define		RISCV_HWPROBE_EXT_ZBKB		(1 << 7)
> >> > +#define		RISCV_HWPROBE_EXT_ZBKC		(1 << 8)
> >> > +#define		RISCV_HWPROBE_EXT_ZBKX		(1 << 9)
> >> > +#define		RISCV_HWPROBE_EXT_ZKND		(1 << 10)
> >> > +#define		RISCV_HWPROBE_EXT_ZKNE		(1 << 11)
> >> > +#define		RISCV_HWPROBE_EXT_ZKNH		(1 << 12)
> >> > +#define		RISCV_HWPROBE_EXT_ZKR		(1 << 13)
> >> > +#define		RISCV_HWPROBE_EXT_ZKSED		(1 << 14)
> >> > +#define		RISCV_HWPROBE_EXT_ZKSH		(1 << 15)
> >> > +#define		RISCV_HWPROBE_EXT_ZKT		(1 << 16)
> >> >  #define RISCV_HWPROBE_KEY_CPUPERF_0	5
> >> >  #define		RISCV_HWPROBE_MISALIGNED_UNKNOWN	(0 << 0)
> >> >  #define		RISCV_HWPROBE_MISALIGNED_EMULATED	(1 << 0)
> >> > diff --git a/arch/riscv/kernel/sys_riscv.c 
> >> > b/arch/riscv/kernel/sys_riscv.c
> >> > index 26ef5526bfb4..df15926196b6 100644
> >> > --- a/arch/riscv/kernel/sys_riscv.c
> >> > +++ b/arch/riscv/kernel/sys_riscv.c
> >> > @@ -145,20 +145,28 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe 
> >> > *pair,
> >> >  	for_each_cpu(cpu, cpus) {
> >> >  		struct riscv_isainfo *isainfo = &hart_isa[cpu];
> >> > 
> >> > -		if (riscv_isa_extension_available(isainfo->isa, ZBA))
> >> > -			pair->value |= RISCV_HWPROBE_EXT_ZBA;
> >> > -		else
> >> > -			missing |= RISCV_HWPROBE_EXT_ZBA;
> >> > -
> >> > -		if (riscv_isa_extension_available(isainfo->isa, ZBB))
> >> > -			pair->value |= RISCV_HWPROBE_EXT_ZBB;
> >> > -		else
> >> > -			missing |= RISCV_HWPROBE_EXT_ZBB;
> >> > -
> >> > -		if (riscv_isa_extension_available(isainfo->isa, ZBS))
> >> > -			pair->value |= RISCV_HWPROBE_EXT_ZBS;
> >> > -		else
> >> > -			missing |= RISCV_HWPROBE_EXT_ZBS;
> >> > +#define SET_HWPROBE_EXT_PAIR(ext)					\
> >> > +		do {							\
> >> > +			if (riscv_isa_extension_available(isainfo->isa, ext)) \
> >> > +				pair->value |= RISCV_HWPROBE_EXT_## ext; \
> >> > +			else						\
> >> > +				missing |= RISCV_HWPROBE_EXT_## ext;	\
> >> > +		} while (false)						\
> >> > +
> >> > +		SET_HWPROBE_EXT_PAIR(ZBA);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBB);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBC);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBS);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBKB);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBKC);
> >> > +		SET_HWPROBE_EXT_PAIR(ZBKX);
> >> > +		SET_HWPROBE_EXT_PAIR(ZKND);
> >> > +		SET_HWPROBE_EXT_PAIR(ZKNE);
> >> > +		SET_HWPROBE_EXT_PAIR(ZKNH);
> >> > +		SET_HWPROBE_EXT_PAIR(ZKR);
> >> 
> >> Does the presence of a HWPROBE_EXT bit imply that userspace software can
> >> actually directly use the described feature?  If so, we should probably
> >> not set ZKR unless mseccfg.USEED=1.
> >
> > mseccfg is MRW, so only accessible from M-mode only afaiu. So I don't
> > think we would be able to check that from Linux in S-mode.
> 
> Check directly, no, but your patch already makes the assumption that
> mseccfg.SSEED=1 if zkr is present in the device tree.  Which is fine as long
> as that contract is documented somewhere (presumably, the device tree
> binding; some of the language in the RVA22U64 profile spec implies USEED=0,
> but linux does not require profiles and they don't exist for rv32).
> 
> If we want U-mode behavior to be discoverable and/or predictable, we have
> three good options:

Thanks for the suggestions.

> Simplest: Document that we expect USEED=0 or USEED=1.  Set zkr appropriately
> in hwprobe.
> 
> Most flexible: Work with the SBI people to add a SBI_EXT_FWFEATURE for USEED,
> as well as defining the value on kernel entry.  Expose this via hwprobe and
> a new prctl.

I'd like to go down that route, but this depends on [1] to get
accepted/merged first.

Would it make sense to go with only documenting the USEED expectation
for now and then move to the more flexible FWFEATURE SBI approach?

Cheers,
Samuel.

[1] https://lists.riscv.org/g/tech-prs/message/540