Re: [PATCH -next v17 10/20] riscv: Allocate user's vector context in the first-use trap

Heiko Stübner
2023-03-31T13:09:18Z

Am Montag, 27. März 2023, 18:49:30 CEST schrieb Andy Chiu:
> Vector unit is disabled by default for all user processes. Thus, a
> process will take a trap (illegal instruction) into kernel at the first
> time when it uses Vector. Only after then, the kernel allocates V
> context and starts take care of the context for that user process.
>
> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Link: https://lore.kernel.org/r/3923eeee-e4dc-0911-40bf-84c34aee962d@linaro.org
> Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> ---
> arch/riscv/include/asm/insn.h | 29 +++++++++++
> arch/riscv/include/asm/vector.h | 2 +
> arch/riscv/kernel/traps.c | 26 +++++++++-
> arch/riscv/kernel/vector.c | 90 +++++++++++++++++++++++++++++++++
> 4 files changed, 145 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
> index 8d5c84f2d5ef..4e1505cef8aa 100644
> --- a/arch/riscv/include/asm/insn.h
> +++ b/arch/riscv/include/asm/insn.h
> @@ -137,6 +137,26 @@
> #define RVG_OPCODE_JALR 0x67
> #define RVG_OPCODE_JAL 0x6f
> #define RVG_OPCODE_SYSTEM 0x73
> +#define RVG_SYSTEM_CSR_OFF 20
> +#define RVG_SYSTEM_CSR_MASK GENMASK(12, 0)

The CSR instructions are all I-type instructions it seems, but I do
understand where this is coming from, as for CSRs this is not an IMM
but an unsigned value

> +/* parts of opcode for RVF, RVD and RVQ */
> +#define RVFDQ_FL_FS_WIDTH_OFF 12
> +#define RVFDQ_FL_FS_WIDTH_MASK GENMASK(3, 0)
> +#define RVFDQ_FL_FS_WIDTH_W 2
> +#define RVFDQ_FL_FS_WIDTH_D 3
> +#define RVFDQ_LS_FS_WIDTH_Q 4
> +#define RVFDQ_OPCODE_FL 0x07
> +#define RVFDQ_OPCODE_FS 0x27

> +/* parts of opcode for RVV */
> +#define RVV_OPCODE_VECTOR 0x57
> +#define RVV_VL_VS_WIDTH_8 0
> +#define RVV_VL_VS_WIDTH_16 5
> +#define RVV_VL_VS_WIDTH_32 6
> +#define RVV_VL_VS_WIDTH_64 7
> +#define RVV_OPCODE_VL RVFDQ_OPCODE_FL
> +#define RVV_OPCODE_VS RVFDQ_OPCODE_FS

Same issue for those (FL_FS + VL_VS), but those even don't declare
being part of any *-type scheme in the spec.

So all of them mix content-definitions (RV*_OPCODE_*) with structure-
definitions and right now I don't know how to feel about that ;-) .

On the one hand I like the original separation, but on the other hand it
doesn't feel useful to put these single-use definitions somewhere above.

> /* parts of opcode for RVC*/
> #define RVC_OPCODE_C0 0x0
> @@ -304,6 +324,15 @@ static __always_inline bool riscv_insn_is_branch(u32 code)
> (RVC_X(x_, RVC_B_IMM_7_6_OPOFF, RVC_B_IMM_7_6_MASK) << RVC_B_IMM_7_6_OFF) | \
> (RVC_IMM_SIGN(x_) << RVC_B_IMM_SIGN_OFF); })
>
> +#define RVG_EXTRACT_SYSTEM_CSR(x) \
> + ({typeof(x) x_ = (x); RV_X(x_, RVG_SYSTEM_CSR_OFF, RVG_SYSTEM_CSR_MASK); })
> +
> +#define RVFDQ_EXTRACT_FL_FS_WIDTH(x) \
> + ({typeof(x) x_ = (x); RV_X(x_, RVFDQ_FL_FS_WIDTH_OFF, \
> + RVFDQ_FL_FS_WIDTH_MASK); })
> +
> +#define RVV_EXRACT_VL_VS_WIDTH(x) RVFDQ_EXTRACT_FL_FS_WIDTH(x)
> +
> /*
> * Get the immediate from a J-type instruction.
> *

> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 03582e2ade83..ea59f32adf46 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -4,9 +4,19 @@
> * Author: Andy Chiu <andy.chiu@sifive.com>
> */
> #include <linux/export.h>
> +#include <linux/sched/signal.h>
> +#include <linux/types.h>
> +#include <linux/slab.h>
> +#include <linux/sched.h>
> +#include <linux/uaccess.h>
>
> +#include <asm/thread_info.h>
> +#include <asm/processor.h>
> +#include <asm/insn.h>
> #include <asm/vector.h>
> #include <asm/csr.h>
> +#include <asm/ptrace.h>
> +#include <asm/bug.h>
>
> unsigned long riscv_v_vsize __read_mostly;
> EXPORT_SYMBOL_GPL(riscv_v_vsize);
> @@ -18,3 +28,83 @@ void riscv_v_setup_vsize(void)
> riscv_v_vsize = csr_read(CSR_VLENB) * 32;
> riscv_v_disable();
> }
> +
> +static bool insn_is_vector(u32 insn_buf)
> +{
> + u32 opcode = insn_buf & __INSN_OPCODE_MASK;
> + bool is_vector = false;
> + u32 width, csr;
> +
> + /*
> + * All V-related instructions, including CSR operations are 4-Byte. So,
> + * do not handle if the instruction length is not 4-Byte.
> + */
> + if (unlikely(GET_INSN_LENGTH(insn_buf) != 4))
> + return false;
> +
> + switch (opcode) {
> + case RVV_OPCODE_VECTOR:
> + is_vector = true;
> + break;
> + case RVV_OPCODE_VL:
> + case RVV_OPCODE_VS:
> + width = RVV_EXRACT_VL_VS_WIDTH(insn_buf);
> + if (width == RVV_VL_VS_WIDTH_8 || width == RVV_VL_VS_WIDTH_16 ||
> + width == RVV_VL_VS_WIDTH_32 || width == RVV_VL_VS_WIDTH_64)
> + is_vector = true;
> + break;
> + case RVG_OPCODE_SYSTEM:
> + csr = RVG_EXTRACT_SYSTEM_CSR(insn_buf);
> + if ((csr >= CSR_VSTART && csr <= CSR_VCSR) ||
> + (csr >= CSR_VL && csr <= CSR_VLENB))
> + is_vector = true;
> + break;
> + }

blank line?

> + return is_vector;

I guess a matter of style-preference, but the other option would be to
just return true from inside the case elements, and simply false here.

> +}
> +
> +static int riscv_v_thread_zalloc(void)
> +{
> + void *datap;
> +
> + datap = kzalloc(riscv_v_vsize, GFP_KERNEL);
> + if (!datap)
> + return -ENOMEM;

blank line?

> + current->thread.vstate.datap = datap;
> + memset(&current->thread.vstate, 0, offsetof(struct __riscv_v_ext_state,
> + datap));
> + return 0;
> +}
> +
> +bool riscv_v_first_use_handler(struct pt_regs *regs)
> +{
> + u32 __user *epc = (u32 __user *)regs->epc;
> + u32 insn = (u32)regs->badaddr;
> +
> + /* If V has been enabled then it is not the first-use trap */
> + if (riscv_v_vstate_query(regs))
> + return false;
> +
> + /* Get the instruction */
> + if (!insn) {
> + if (__get_user(insn, epc))
> + return false;
> + }

blank?

> + /* Filter out non-V instructions */
> + if (!insn_is_vector(insn))
> + return false;
> +
> + /* Sanity check. datap should be null by the time of the first-use trap */
> + WARN_ON(current->thread.vstate.datap);

blank?

> + /*
> + * Now we sure that this is a V instruction. And it executes in the
> + * context where VS has been off. So, try to allocate the user's V
> + * context and resume execution.
> + */
> + if (riscv_v_thread_zalloc()) {
> + force_sig(SIGKILL);
> + return true;
> + }
> + riscv_v_vstate_on(regs);
> + return true;
> +}
>

in any case,

Acked-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>

Heiko