[PATCH] lib: sbi: Enable Control Transfer Records (CTR) Ext using xstateen.

[Anup Patel]

Hi Rajnesh,

On Wed, May 29, 2024 at 8:32 PM Rajnesh Kanwal <rkanwal at rivosinc.com> wrote:
>
> The Control Transfer Records (CTR) extension provides a method to
> record a limited branch history in register-accessible internal chip
> storage.
>
> This extension is similar to Arch LBR in x86 and BRBE in ARM.
> The Extension has been stable and the latest release can be found here
> https://github.com/riscv/riscv-control-transfer-records/release
>
> Signed-off-by: Rajnesh Kanwal <rkanwal at rivosinc.com>

We will be needing a SBI FWFT extension feature to explicitly
enable/disable CTR because:
1) Hypervisors (such as KVM) needs a way to know when
Guest/VM starts/stops using CTR. This information will allow
hypervisors to optimize Guest/VM world switch when CTR is
available.
2) Keeping is CTR enabled by default opens a side-channel
between multiple OpenSBI domains or between multiple
Guest/VM.

Based on the above, I suggest you propose a new feature for
SBI FWFT extension in PRS TG and update this series accordingly.

Regards,
Anup

> ---
>  include/sbi/riscv_encoding.h | 13 +++++++++++++
>  include/sbi/sbi_hart.h       |  4 ++++
>  lib/sbi/sbi_hart.c           | 13 +++++++++++++
>  3 files changed, 30 insertions(+)
>
> diff --git a/include/sbi/riscv_encoding.h b/include/sbi/riscv_encoding.h
> index d914828..eb3361c 100644
> --- a/include/sbi/riscv_encoding.h
> +++ b/include/sbi/riscv_encoding.h
> @@ -357,6 +357,17 @@
>  #define CSR_SSTATEEN2                  0x10E
>  #define CSR_SSTATEEN3                  0x10F
>
> +/* Machine-Level Control transfer records CSRs */
> +#define CSR_MCTRCTL                     0x34e
> +
> +/* Supervisor-Level Control transfer records CSRs */
> +#define CSR_SCTRCTL                     0x14e
> +#define CSR_SCTRSTATUS                  0x14f
> +#define CSR_SCTRDEPTH                   0x15f
> +
> +/* VS-Level Control transfer records CSRs */
> +#define CSR_VSCTRCTL                    0x24e
> +
>  /* ===== Hypervisor-level CSRs ===== */
>
>  /* Hypervisor Trap Setup (H-extension) */
> @@ -773,6 +784,8 @@
>  #define SMSTATEEN0_CS                  (_ULL(1) << SMSTATEEN0_CS_SHIFT)
>  #define SMSTATEEN0_FCSR_SHIFT          1
>  #define SMSTATEEN0_FCSR                        (_ULL(1) << SMSTATEEN0_FCSR_SHIFT)
> +#define SMSTATEEN0_CTR_SHIFT           54
> +#define SMSTATEEN0_CTR                 (_ULL(1) << SMSTATEEN0_CTR_SHIFT)
>  #define SMSTATEEN0_CONTEXT_SHIFT       57
>  #define SMSTATEEN0_CONTEXT             (_ULL(1) << SMSTATEEN0_CONTEXT_SHIFT)
>  #define SMSTATEEN0_IMSIC_SHIFT         58
> diff --git a/include/sbi/sbi_hart.h b/include/sbi/sbi_hart.h
> index cc78eec..c4eddb4 100644
> --- a/include/sbi/sbi_hart.h
> +++ b/include/sbi/sbi_hart.h
> @@ -63,6 +63,10 @@ enum sbi_hart_extensions {
>         SBI_HART_EXT_SSCSRIND,
>         /** Hart has Ssccfg extension */
>         SBI_HART_EXT_SSCCFG,
> +       /** HART has CTR M-mode CSRs */
> +       SBI_HART_EXT_SMCTR,
> +       /** HART has CTR S-mode CSRs */
> +       SBI_HART_EXT_SSCTR,
>
>         /** Maximum index of Hart extension */
>         SBI_HART_EXT_MAX,
> diff --git a/lib/sbi/sbi_hart.c b/lib/sbi/sbi_hart.c
> index cd974cc..db2f157 100644
> --- a/lib/sbi/sbi_hart.c
> +++ b/lib/sbi/sbi_hart.c
> @@ -105,6 +105,11 @@ static void mstatus_init(struct sbi_scratch *scratch)
>                 else
>                         mstateen_val &= ~(SMSTATEEN0_SVSLCT);
>
> +               if (sbi_hart_has_extension(scratch, SBI_HART_EXT_SMCTR))
> +                       mstateen_val |= SMSTATEEN0_CTR;
> +               else
> +                       mstateen_val &= ~SMSTATEEN0_CTR;
> +
>                 csr_write(CSR_MSTATEEN0, mstateen_val);
>  #if __riscv_xlen == 32
>                 csr_write(CSR_MSTATEEN0H, mstateen_val >> 32);
> @@ -668,6 +673,8 @@ const struct sbi_hart_ext_data sbi_hart_ext[] = {
>         __SBI_HART_EXT_DATA(smcdeleg, SBI_HART_EXT_SMCDELEG),
>         __SBI_HART_EXT_DATA(sscsrind, SBI_HART_EXT_SSCSRIND),
>         __SBI_HART_EXT_DATA(ssccfg, SBI_HART_EXT_SSCCFG),
> +       __SBI_HART_EXT_DATA(smctr, SBI_HART_EXT_SMCTR),
> +       __SBI_HART_EXT_DATA(ssctr, SBI_HART_EXT_SSCTR),
>  };
>
>  _Static_assert(SBI_HART_EXT_MAX == array_size(sbi_hart_ext),
> @@ -921,6 +928,12 @@ __pmp_skip:
>         /* Save trap based detection of Zicntr */
>         has_zicntr = sbi_hart_has_extension(scratch, SBI_HART_EXT_ZICNTR);
>
> +       /* Detect if hart has Control Transfer Record CSRs */
> +       csr_read_allowed(CSR_MCTRCTL, (unsigned long)&trap);
> +       if (!trap.cause)
> +               __sbi_hart_update_extension(hfeatures, SBI_HART_EXT_SMCTR,
> +                                           true);
> +
>         /* Let platform populate extensions */
>         rc = sbi_platform_extensions_init(sbi_platform_thishart_ptr(),
>                                           hfeatures);
> --
> 2.34.1
>
>
> --
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