On Fri, Apr 12, 2024 at 11:17 AM Charlie Jenkins charlie@rivosinc.com wrote:
On Fri, Apr 12, 2024 at 10:05:21AM -0700, Evan Green wrote:
On Thu, Apr 11, 2024 at 9:12 PM Charlie Jenkins charlie@rivosinc.com wrote:
Add a new hwprobe key "RISCV_HWPROBE_KEY_VENDOR_EXT_0" which allows userspace to probe for the new RISCV_ISA_VENDOR_EXT_XTHEADVECTOR vendor extension.
Signed-off-by: Charlie Jenkins charlie@rivosinc.com
arch/riscv/include/asm/hwprobe.h | 4 +-- arch/riscv/include/uapi/asm/hwprobe.h | 10 +++++- arch/riscv/kernel/sys_hwprobe.c | 59 +++++++++++++++++++++++++++++++++-- 3 files changed, 68 insertions(+), 5 deletions(-)
diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h index 630507dff5ea..e68496b4f8de 100644 --- a/arch/riscv/include/asm/hwprobe.h +++ b/arch/riscv/include/asm/hwprobe.h @@ -1,6 +1,6 @@ /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ /*
- Copyright 2023 Rivos, Inc
*/
- Copyright 2023-2024 Rivos, Inc
#ifndef _ASM_HWPROBE_H @@ -8,7 +8,7 @@
#include <uapi/asm/hwprobe.h>
-#define RISCV_HWPROBE_MAX_KEY 6 +#define RISCV_HWPROBE_MAX_KEY 7
static inline bool riscv_hwprobe_key_is_valid(__s64 key) { diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h index 9f2a8e3ff204..6614d3adfc75 100644 --- a/arch/riscv/include/uapi/asm/hwprobe.h +++ b/arch/riscv/include/uapi/asm/hwprobe.h @@ -1,6 +1,6 @@ /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ /*
- Copyright 2023 Rivos, Inc
*/
- Copyright 2023-2024 Rivos, Inc
#ifndef _UAPI_ASM_HWPROBE_H @@ -67,6 +67,14 @@ struct riscv_hwprobe { #define RISCV_HWPROBE_MISALIGNED_UNSUPPORTED (4 << 0) #define RISCV_HWPROBE_MISALIGNED_MASK (7 << 0) #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE 6 +/*
- It is not possible for one CPU to have multiple vendor ids, so each vendor
- has its own vendor extension "namespace". The keys for each vendor starts
- at zero.
- */
+#define RISCV_HWPROBE_KEY_VENDOR_EXT_0 7
- /* T-Head */
+#define RISCV_HWPROBE_VENDOR_EXT_XTHEADVECTOR (1 << 0) /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
/* Flags */ diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c index e0a42c851511..365ce7380443 100644 --- a/arch/riscv/kernel/sys_hwprobe.c +++ b/arch/riscv/kernel/sys_hwprobe.c @@ -69,7 +69,8 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair, if (riscv_isa_extension_available(NULL, c)) pair->value |= RISCV_HWPROBE_IMA_C;
if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
if (has_vector() &&!__riscv_isa_vendor_extension_available(NULL, RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) pair->value |= RISCV_HWPROBE_IMA_V; /*@@ -112,7 +113,8 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair, EXT_KEY(ZACAS); EXT_KEY(ZICOND);
if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) {
if (has_vector() &&!riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) { EXT_KEY(ZVBB); EXT_KEY(ZVBC); EXT_KEY(ZVKB);@@ -139,6 +141,55 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair, pair->value &= ~missing; }
+static void hwprobe_isa_vendor_ext0(struct riscv_hwprobe *pair,
const struct cpumask *cpus)+{
int cpu;u64 missing = 0;pair->value = 0;struct riscv_hwprobe mvendorid = {.key = RISCV_HWPROBE_KEY_MVENDORID,.value = 0};hwprobe_arch_id(&mvendorid, cpus);/* Set value to zero if CPUs in the set do not have the same vendor. */if (mvendorid.value == -1ULL)return;/** Loop through and record vendor extensions that 1) anyone has, and* 2) anyone doesn't have.*/for_each_cpu(cpu, cpus) {struct riscv_isainfo *isavendorinfo = &hart_isa_vendor[cpu];+#define VENDOR_EXT_KEY(ext) \
do { \if (__riscv_isa_vendor_extension_available(isavendorinfo->isa, \RISCV_ISA_VENDOR_EXT_##ext)) \pair->value |= RISCV_HWPROBE_VENDOR_EXT_##ext; \else \missing |= RISCV_HWPROBE_VENDOR_EXT_##ext; \} while (false)/** Only use VENDOR_EXT_KEY() for extensions which can be exposed to userspace,* regardless of the kernel's configuration, as no other checks, besides* presence in the hart_vendor_isa bitmap, are made.*/VENDOR_EXT_KEY(XTHEADVECTOR);+#undef VENDOR_EXT_KEY
Hey Charlie, Thanks for writing this up! At the very least I think the THEAD-specific stuff should probably end up in its own file, otherwise it'll get chaotic with vendors clamoring to add stuff right here.
Great idea!
What do you think about this approach:
- We leave RISCV_HWPROBE_MAX_KEY as the max key for the "generic
world", eg 6-ish
- We define that any key above 0x8000000000000000 is in the vendor
space, so the meaning of the keys depends first on the mvendorid value.
- In the kernel code, each new vendor adds on to a global struct,
which might look something like: struct hwprobe_vendor_space vendor_space[] = { { .mvendorid = VENDOR_THEAD, .max_hwprobe_key = THEAD_MAX_HWPROBE_KEY, // currently 1 or 0x8000000000000001 with what you've got. .hwprobe_fn = thead_hwprobe }, ... };
- A hwprobe_thead.c implements thead_hwprobe(), and is called
whenever the generic hwprobe encounters a key >=0x8000000000000000.
- Generic code for setting up the VDSO can then still call the
vendor-specific hwprobe_fn() repeatedly with an "all CPUs" mask from the base to max_hwprobe_key and set up the cached tables in userspace.
- Since the VDSO data has limited space we may have to cap the number
of vendor keys we cache to be lower than max_hwprobe_key. Since the data itself is not exposed to usermode we can raise this cap later if needed.
I know vendor extensions are kind of the "wild west" of riscv, but in spite of that I want to design a consistent API. The issue I had with having this "vendor space" for exposing vendor extensions was that this is something that is inherently the same for all vendors. I see a vendor space like this more applicable for something like "RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE" where a vendor has a specific value they would like to expose. I do agree that having a vendor space is a good design choice, but I am not convinced that vendor extensions are the proper use-case.
By having RISCV_HWPROBE_KEY_VENDOR_EXT_0 we can expose the vendor extensions in the same way that standard extensions are exposed, with a bitmask representing each extension. If these are instead in the vendor space, each vendor would probably be inclined to introduce a key like RISCV_HWPROBE_KEY_THEAD_EXT_0 that returns a bitmask of all of the thead vendor extensions. This duplicated effort is what I am trying to avoid. The alternative would be that vendors have a separate key for each vendor extension they would like to expose, but that is strictly less efficient than the existing bitmask probing.
Do you think that having the vendor space is appropriate for vendor extensions given my concerns?
I do see what you're going for. It's tidy for a bitmask to just let anyone allocate the next bit, but leaves you with the same problem when a vendor decides they want to expose an enum, or decides they want to expose a bazillion things. I think a generalized version of the approach you've written would be: simply let vendors allocate keys from the same global space we're already using. My worry was that it would turn into an expansive suburban sprawl of mostly dead bits, or in the case of vendor-specific keys, full of "if (mvendor_id() != MINE) return 0;". My hope with the vendored keyspace is it would keep the sprawl from polluting the general array of (hopefully valuable) info with stuff that's likely to become less relevant as time passes. It also lowers the bar a bit to make it easier for vendors to expose bits, as they don't consume global space for everyone for all of time, just themselves.
So yes, personally I'm still in the camp of siloing the vendor stuff off to its own area. -Evan