optimize the memory probing for vector fault-only-first loads.

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 broad 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>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250221155320.59159-2-paolo.savini@embecosm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

1 files changed, 58 insertions, 45 deletions

diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
index 7773df6..71b823d 100644
--- a/target/riscv/vector_helper.c
+++ b/target/riscv/vector_helper.c
@@ -633,47 +633,69 @@ vext_ldff(void *vd, void *v0, target_ulong base, CPURISCVState *env,
     uint32_t esz = 1 << log2_esz;
     uint32_t msize = nf * esz;
     uint32_t vma = vext_vma(desc);
-    target_ulong addr, offset, remain, page_split, elems;
+    target_ulong addr, addr_probe, addr_i, 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;
+    addr = base + ((env->vstart * nf) << log2_esz);
+    page_split = -(addr | TARGET_PAGE_MASK);
+    /* Get number of elements */
+    elems = page_split / msize;
+    if (unlikely(env->vstart + elems >= env->vl)) {
+        elems = env->vl - env->vstart;
+    }
+
+    /* Check page permission/pmp/watchpoint/etc. */
+    flags = probe_access_flags(env, adjust_addr(env, addr), elems * msize,
+                               MMU_DATA_LOAD, mmu_index, true, &host, ra);
+
+    /* If we are crossing a page check also the second page. */
+    if (env->vl > elems) {
+        addr_probe = addr + (elems << log2_esz);
+        flags |= probe_access_flags(env, adjust_addr(env, addr_probe),
+                                    elems * msize, 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_i = adjust_addr(env, base + i * (nf << log2_esz));
+            if (i == 0) {
+                /* Allow fault on first element. */
+                probe_pages(env, addr_i, nf << log2_esz, ra, MMU_DATA_LOAD);
+            } else {
+                remain = nf << log2_esz;
+                while (remain > 0) {
+                    offset = -(addr_i | TARGET_PAGE_MASK);
+
+                    /* Probe nonfault on subsequent elements. */
+                    flags = probe_access_flags(env, addr_i, 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_i = adjust_addr(env, addr_i + offset);
                 }
-                remain -= offset;
-                addr = adjust_addr(env, addr + offset);
             }
         }
     }
@@ -685,15 +707,6 @@ ProbeSuccess:
 
     if (env->vstart < env->vl) {
         if (vm) {
-            /* Calculate the page range of first page */
-            addr = base + ((env->vstart * nf) << log2_esz);
-            page_split = -(addr | TARGET_PAGE_MASK);
-            /* Get number of elements */
-            elems = page_split / msize;
-            if (unlikely(env->vstart + elems >= env->vl)) {
-                elems = env->vl - env->vstart;
-            }
-
             /* Load/store elements in the first page */
             if (likely(elems)) {
                 vext_page_ldst_us(env, vd, addr, elems, nf, max_elems,