diff mbox series

[RFC,1/1,v1] target/riscv: optimize the memory probing for vector fault-only-first loads.

Message ID 20250129144435.82451-2-paolo.savini@embecosm.com (mailing list archive)
State New
Headers show
Series target/riscv: optimize the memory probing for vector fault-only-first loads. | expand

Commit Message

Paolo Savini Jan. 29, 2025, 2:44 p.m. UTC
Fault-only-first loads in the RISC-V vector extension need to update
the vl with the element index that causes an exception.
In order to ensure this the emulation of this instruction used to probe the
memory covered by the load operation with a loop that iterated over each element
so that when a flag was raised it was possible to set the vl to the
corresponding element index.
This loop was executed every time whether an exception happened or not.

This commit removes the per element memory probing from the main execution path
and adds a braod memory probing first. If this probing raises any flag that is
not a watchpoint flag (that per standard is allowed by this instruction) we
proceed with the per element probing to find the index of the element causing
the exception and set vl to such index.

Signed-off-by: Paolo Savini <paolo.savini@embecosm.com>
---
 target/riscv/vector_helper.c | 91 ++++++++++++++++++++++--------------
 1 file changed, 56 insertions(+), 35 deletions(-)
diff mbox series

Patch

diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
index 5386e3b97c..d3ac99c72d 100644
--- a/target/riscv/vector_helper.c
+++ b/target/riscv/vector_helper.c
@@ -635,45 +635,66 @@  vext_ldff(void *vd, void *v0, target_ulong base, CPURISCVState *env,
     uint32_t vma = vext_vma(desc);
     target_ulong addr, offset, remain, page_split, elems;
     int mmu_index = riscv_env_mmu_index(env, false);
+    int flags;
+    void *host;
 
     VSTART_CHECK_EARLY_EXIT(env);
 
-    /* probe every access */
-    for (i = env->vstart; i < env->vl; i++) {
-        if (!vm && !vext_elem_mask(v0, i)) {
-            continue;
-        }
-        addr = adjust_addr(env, base + i * (nf << log2_esz));
-        if (i == 0) {
-            /* Allow fault on first element. */
-            probe_pages(env, addr, nf << log2_esz, ra, MMU_DATA_LOAD);
-        } else {
-            remain = nf << log2_esz;
-            while (remain > 0) {
-                void *host;
-                int flags;
-
-                offset = -(addr | TARGET_PAGE_MASK);
-
-                /* Probe nonfault on subsequent elements. */
-                flags = probe_access_flags(env, addr, offset, MMU_DATA_LOAD,
-                                           mmu_index, true, &host, 0);
-
-                /*
-                 * Stop if invalid (unmapped) or mmio (transaction may fail).
-                 * Do not stop if watchpoint, as the spec says that
-                 * first-fault should continue to access the same
-                 * elements regardless of any watchpoint.
-                 */
-                if (flags & ~TLB_WATCHPOINT) {
-                    vl = i;
-                    goto ProbeSuccess;
-                }
-                if (remain <= offset) {
-                    break;
+    uint32_t probe_size = env->vl << log2_esz;
+    addr = base + ((env->vstart * nf) << log2_esz);
+    page_split = -(addr | TARGET_PAGE_MASK);
+    /* Get number of elements */
+    probe_size = page_split / msize;
+    if (unlikely(env->vstart + probe_size >= env->vl)) {
+        probe_size = env->vl - env->vstart;
+    }
+
+    /* Check page permission/pmp/watchpoint/etc. */
+    flags = probe_access_flags(env, adjust_addr(env, addr), probe_size,
+                               MMU_DATA_LOAD, mmu_index, true, &host, ra);
+
+    /* If we are crossing a page check also the second page. */
+    if (env->vl > probe_size) {
+        addr = addr + (probe_size << log2_esz);
+        flags |= probe_access_flags(env, adjust_addr(env, addr), probe_size,
+                                    MMU_DATA_LOAD, mmu_index, true, &host, ra);
+    }
+
+    if (flags & ~TLB_WATCHPOINT) {
+        /* probe every access */
+        for (i = env->vstart; i < env->vl; i++) {
+            if (!vm && !vext_elem_mask(v0, i)) {
+                continue;
+            }
+            addr = adjust_addr(env, base + i * (nf << log2_esz));
+            if (i == 0) {
+                /* Allow fault on first element. */
+                probe_pages(env, addr, nf << log2_esz, ra, MMU_DATA_LOAD);
+            } else {
+                remain = nf << log2_esz;
+                while (remain > 0) {
+                    offset = -(addr | TARGET_PAGE_MASK);
+
+                    /* Probe nonfault on subsequent elements. */
+                    flags = probe_access_flags(env, addr, offset, MMU_DATA_LOAD,
+                                               mmu_index, true, &host, 0);
+
+                    /*
+                     * Stop if invalid (unmapped) or mmio (transaction may
+                     * fail). Do not stop if watchpoint, as the spec says that
+                     * first-fault should continue to access the same
+                     * elements regardless of any watchpoint.
+                     */
+                    if (flags & ~TLB_WATCHPOINT) {
+                        vl = i;
+                        goto ProbeSuccess;
+                    }
+                    if (remain <= offset) {
+                        break;
+                    }
+                    remain -= offset;
+                    addr = adjust_addr(env, addr + offset);
                 }
-                remain -= offset;
-                addr = adjust_addr(env, addr + offset);
             }
         }
     }