On Fri, Aug 25, 2023 at 04:11:38PM -0700, Evan Green wrote:
> In /proc/cpuinfo, most of the information we show for each processor is
> specific to that hart: marchid, mvendorid, mimpid, processor, hart,
> compatible, and the mmu size. But the ISA string gets filtered through a
> lowest common denominator mask, so that if one CPU is missing an ISA
> extension, no CPUs will show it.
>
> Now that we track the ISA extensions for each hart, let's report ISA
> extension info accurately per-hart in /proc/cpuinfo. We cannot change
> the "isa:" line, as usermode may be relying on that line to show only
> the common set of extensions supported across all harts. Add a new "hart
> isa" line instead, which reports the true set of extensions for that
> hart.
>
> Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
> Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
>
> ---
>
> Changes in v5:
> - Documentation changes (only) (Conor)
>
> Changes in v4:
> - Documentation: Made the underline match the text line (Conor)
> - Documentation: hanged "in question" to "being described" (Andrew)
>
> Changes in v3:
> - Add some documentation (Conor)
>
> Changes in v2:
> - Added new "hart isa" line rather than altering behavior of existing
> "isa" line (Conor, Palmer)
>
> Documentation/riscv/uabi.rst | 12 ++++++++++++
> arch/riscv/kernel/cpu.c | 22 ++++++++++++++++++----
> 2 files changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/riscv/uabi.rst b/Documentation/riscv/uabi.rst
> index 8960fac42c40..a46017f57db2 100644
> --- a/Documentation/riscv/uabi.rst
> +++ b/Documentation/riscv/uabi.rst
> @@ -42,6 +42,18 @@ An example string following the order is::
>
> rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
>
> +"isa" vs "hart isa" lines in /proc/cpuinfo
> +------------------------------------------
> +
> +The "isa" line in /proc/cpuinfo describes the lowest common denominator of
> +RISC-V ISA extensions recognized by the kernel and implemented on all harts. The
> +"hart isa" line, in contrast, describes the set of extensions recognized by the
> +kernel on the particular hart being described, even if those extensions may not
> +be present on all harts in the system. In both cases, the presence of a feature
> +in these lines guarantees only that the hardware has the described capability.
> +Additional kernel support or policy control changes may be required before a
> +feature is fully usable by userspace programs.
These last words imply that extensions listed in /proc/cpuinfo are all
potentially usable by userspace, but we also advertise extensions which
are only usable in S-mode (which is fine because of VMs). We should just
make that clear here.
Also, I frequently bounce between the words 'feature' and 'extension',
but, for documentation, I think we should try to be consistent. Or, we
could write somewhere that 'feature' == 'extension'.
Thanks,
drew
> +
> Misaligned accesses
> -------------------
>
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index 7b793c4321bb..100fb382b450 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -197,9 +197,8 @@ arch_initcall(riscv_cpuinfo_init);
>
> #ifdef CONFIG_PROC_FS
>
> -static void print_isa(struct seq_file *f)
> +static void print_isa(struct seq_file *f, const unsigned long *isa_bitmap)
> {
> - seq_puts(f, "isa\t\t: ");
>
> if (IS_ENABLED(CONFIG_32BIT))
> seq_write(f, "rv32", 4);
> @@ -207,7 +206,7 @@ static void print_isa(struct seq_file *f)
> seq_write(f, "rv64", 4);
>
> for (int i = 0; i < riscv_isa_ext_count; i++) {
> - if (!__riscv_isa_extension_available(NULL, riscv_isa_ext[i].id))
> + if (!__riscv_isa_extension_available(isa_bitmap, riscv_isa_ext[i].id))
> continue;
>
> /* Only multi-letter extensions are split by underscores */
> @@ -271,7 +270,15 @@ static int c_show(struct seq_file *m, void *v)
>
> seq_printf(m, "processor\t: %lu\n", cpu_id);
> seq_printf(m, "hart\t\t: %lu\n", cpuid_to_hartid_map(cpu_id));
> - print_isa(m);
> +
> + /*
> + * For historical raisins, the isa: line is limited to the lowest common
> + * denominator of extensions supported across all harts. A true list of
> + * extensions supported on this hart is printed later in the hart_isa:
> + * line.
> + */
> + seq_puts(m, "isa\t\t: ");
> + print_isa(m, NULL);
> print_mmu(m);
>
> if (acpi_disabled) {
> @@ -287,6 +294,13 @@ static int c_show(struct seq_file *m, void *v)
> seq_printf(m, "mvendorid\t: 0x%lx\n", ci->mvendorid);
> seq_printf(m, "marchid\t\t: 0x%lx\n", ci->marchid);
> seq_printf(m, "mimpid\t\t: 0x%lx\n", ci->mimpid);
> +
> + /*
> + * Print the ISA extensions specific to this hart, which may show
> + * additional extensions not present across all harts.
> + */
> + seq_puts(m, "hart isa\t: ");
> + print_isa(m, hart_isa[cpu_id].isa);
> seq_puts(m, "\n");
>
> return 0;
> --
> 2.34.1
>
