Merge patch series "Svvptc extension to remove preventive sfence.vma"

Alexandre Ghiti <alexghiti@rivosinc.com> says:

In RISC-V, after a new mapping is established, a sfence.vma needs to be
emitted for different reasons:

- if the uarch caches invalid entries, we need to invalidate it otherwise
  we would trap on this invalid entry,
- if the uarch does not cache invalid entries, a reordered access could fail
  to see the new mapping and then trap (sfence.vma acts as a fence).

We can actually avoid emitting those (mostly) useless and costly sfence.vma
by handling the traps instead:

- for new kernel mappings: only vmalloc mappings need to be taken care of,
  other new mapping are rare and already emit the required sfence.vma if
  needed.
  That must be achieved very early in the exception path as explained in
  patch 3, and this also fixes our fragile way of dealing with vmalloc faults.

- for new user mappings: Svvptc makes update_mmu_cache() a no-op but we can
  take some gratuitous page faults (which are very unlikely though).

Patch 1 and 2 introduce Svvptc extension probing.

On our uarch that does not cache invalid entries and a 6.5 kernel, the
gains are measurable:

* Kernel boot:                  6%
* ltp - mmapstress01:           8%
* lmbench - lat_pagefault:      20%
* lmbench - lat_mmap:           5%

Here are the corresponding numbers of sfence.vma emitted:

* Ubuntu boot to login:
Before: ~630k sfence.vma
After:  ~200k sfence.vma

* ltp - mmapstress01
Before: ~45k
After:  ~6.3k

* lmbench - lat_pagefault
Before: ~665k
After:   832 (!)

* lmbench - lat_mmap
Before: ~546k
After:   718 (!)

Thanks to Ved and Matt Evans for triggering the discussion that led to
this patchset!

* b4-shazam-merge:
  riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc
  riscv: Stop emitting preventive sfence.vma for new vmalloc mappings
  dt-bindings: riscv: Add Svvptc ISA extension description
  riscv: Add ISA extension parsing for Svvptc

Link: https://lore.kernel.org/r/20240717060125.139416-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

2 files changed, 15 insertions, 0 deletions

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 0e56725377a4..206bae58ed6c 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -37,6 +37,8 @@
 
 #include "../kernel/head.h"
 
+u64 new_vmalloc[NR_CPUS / sizeof(u64) + 1];
+
 struct kernel_mapping kernel_map __ro_after_init;
 EXPORT_SYMBOL(kernel_map);
 #ifdef CONFIG_XIP_KERNEL
diff --git a/arch/riscv/mm/pgtable.c b/arch/riscv/mm/pgtable.c
index 533ec9055fa0..4ae67324f992 100644
--- a/arch/riscv/mm/pgtable.c
+++ b/arch/riscv/mm/pgtable.c
@@ -9,6 +9,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 			  unsigned long address, pte_t *ptep,
 			  pte_t entry, int dirty)
 {
+	asm goto(ALTERNATIVE("nop", "j %l[svvptc]", 0, RISCV_ISA_EXT_SVVPTC, 1)
+		 : : : : svvptc);
+
 	if (!pte_same(ptep_get(ptep), entry))
 		__set_pte_at(vma->vm_mm, ptep, entry);
 	/*
@@ -16,6 +19,16 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 	 * the case that the PTE changed and the spurious fault case.
 	 */
 	return true;
+
+svvptc:
+	if (!pte_same(ptep_get(ptep), entry)) {
+		__set_pte_at(vma->vm_mm, ptep, entry);
+		/* Here only not svadu is impacted */
+		flush_tlb_page(vma, address);
+		return true;
+	}
+
+	return false;
 }
 
 int ptep_test_and_clear_young(struct vm_area_struct *vma,