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Date:   Wed, 28 Jun 2023 09:25:02 -0400
From:   "Stefan O'Rear" <sorear@fastmail.com>
To:     "Samuel Ortiz" <sameo@rivosinc.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>,
        =?UTF-8?Q?Bj=C3=B6rn_T=C3=B6pel?= <bjorn@rivosinc.com>,
        "Evan Green" <evan@rivosinc.com>
Subject: Re: [PATCH 2/3] RISC-V: hwprobe: Expose Zbc and the scalar crypto extensions
Content-Type: text/plain
Precedence: bulk

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:

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.

Most robust: Add an illegal instruction handler for CSR_SEED that grabs 16
bits from the kernel entropy pool.  Expose zkr in hwprobe unconditionally.

-s

> Cheers,
> Samuel.