[PATCH 5/8] RFC: accel/tcg: allow cross-page goto_tb chaining in user-only builds

[Matt Turner <mattst88@gmail.com>]

translator_use_goto_tb() refuses to chain unless the destination is on the
same page as the start of the TB. For guests whose text is much larger than
a page this is expensive: an emulated alpha gcc compiling a 255k line
translation unit takes the indirect dispatch path for 8.4 billion of its
34.2 billion TB exits, and a large share of those are ordinary direct
branches that simply crossed an 8 KiB page boundary.

The restriction was made unconditional by d3a2a1d803 ("accel/tcg:
Introduce translator_use_goto_tb"), whose rationale was:

    Various targets avoid the page crossing test for CONFIG_USER_ONLY,
    but that is wrong: mmap and mprotect can change page permissions.

That is true, but in user-only builds the invalidation path already covers
it. There are no page tables: every mmap, mprotect and munmap reaches
page_set_flags(), which calls tb_invalidate_phys_range() whenever the flags
actually change, and tb_phys_invalidate() calls tb_jmp_unlink() to reset
incoming jumps. A chained cross-page jump is therefore broken whenever the
destination page's permissions change. This is not true in system mode,
where TBs are keyed by physical address and a page table change invalidates
nothing, so the restriction is kept there.

Add tests/tcg/alpha/test-xpage-chain.c to cover the hazard directly. It
places a direct branch near the end of one page targeting the next page,
runs it 200000 times so the chain is established, then checks that
mprotect(PROT_NONE) makes the next call fault, and that remapping the page
with different code runs the new code rather than a stale translation.

The test detects the hazard it is meant to detect: with the
tb_invalidate_phys_range() call in page_set_flags() commented out, it fails
both phases, executing page B after PROT_NONE and returning the stale
result.

Measured with qemu-alpha running an emulated alpha gcc 16.2.0 compiling the
SQLite 3.45.1 amalgamation on an x86-64 host, LTO build, on top of the
preceding patches:

    before: 890,713,633,237 instructions
    after:  869,178,598,378 instructions   -2.42%

    before: 84.44s wall clock
    after:  80.49s wall clock              -4.68%

Note that this is worth more in time than in instructions, the reverse of
the preceding patch: a chained jump replaces a cache probe whose loads can
miss, so the instructions it removes are more expensive than average.

Measured before the inline jump cache probe, when a missed chain cost a
helper call rather than an inline probe, the same change was worth -7.9%.

RFC because this reverses a deliberate decision and the reasoning above
wants review from someone who knows the invalidation paths better than I
do.

Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 accel/tcg/translator.c             |  12 +++-
 tests/tcg/alpha/Makefile.target    |   2 +-
 tests/tcg/alpha/test-xpage-chain.c | 111 +++++++++++++++++++++++++++++
 3 files changed, 123 insertions(+), 2 deletions(-)
 create mode 100644 tests/tcg/alpha/test-xpage-chain.c

diff --git ./accel/tcg/translator.c ./accel/tcg/translator.c
index 85bb21e911..3eab9f570d 100644
--- ./accel/tcg/translator.c
+++ ./accel/tcg/translator.c
@@ -108,6 +108,17 @@ static void gen_tb_end(const TranslationBlock *tb, uint32_t cflags,
 
 bool translator_is_same_page(const DisasContextBase *db, vaddr addr)
 {
+    /*
+     * In user-only mode there are no page tables.  Every mmap, mprotect and
+     * munmap goes through page_set_flags(), which calls
+     * tb_invalidate_phys_range() whenever the flags actually change, and
+     * tb_phys_invalidate() unlinks incoming jumps.  A cross-page link is
+     * therefore broken whenever the destination page's permissions change,
+     * so the same-page restriction is not needed.
+     */
+    if (IS_ENABLED(CONFIG_USER_ONLY)) {
+        return true;
+    }
     return ((addr ^ db->pc_first) & TARGET_PAGE_MASK) == 0;
 }
 
@@ -118,7 +129,6 @@ bool translator_use_goto_tb(DisasContextBase *db, vaddr dest)
         return false;
     }
 
-    /* Check for the dest on the same page as the start of the TB.  */
     return translator_is_same_page(db, dest);
 }
 
diff --git ./tests/tcg/alpha/Makefile.target ./tests/tcg/alpha/Makefile.target
index 36d8ed1eae..eee986bab6 100644
--- ./tests/tcg/alpha/Makefile.target
+++ ./tests/tcg/alpha/Makefile.target
@@ -5,7 +5,7 @@
 ALPHA_SRC=$(SRC_PATH)/tests/tcg/alpha
 VPATH+=$(ALPHA_SRC)
 
-ALPHA_TESTS=hello-alpha test-cond test-cmov test-ovf test-cvttq
+ALPHA_TESTS=hello-alpha test-cond test-cmov test-ovf test-cvttq test-xpage-chain
 TESTS+=$(ALPHA_TESTS)
 
 test-cmov: EXTRA_CFLAGS=-DTEST_CMOV
diff --git ./tests/tcg/alpha/test-xpage-chain.c ./tests/tcg/alpha/test-xpage-chain.c
new file mode 100644
index 0000000000..7916d544af
--- /dev/null
+++ ./tests/tcg/alpha/test-xpage-chain.c
@@ -0,0 +1,111 @@
+/*
+ * Cross-page TB chaining hazard test.
+ *
+ * Phase 1: a direct branch (br) near the end of page A targets page B.
+ *          Run it enough times that QEMU chains TB_A -> TB_B.
+ * Phase 2: mprotect page B away. Re-running must fault.
+ * Phase 3: remap page B with different code. Re-running must execute the
+ *          NEW code, not a stale chained translation of the old code.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <setjmp.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#define PS 8192
+
+static sigjmp_buf jb;
+/*
+ * Written by the SIGSEGV handler and read by main(), so it must not be
+ * cached in a register across the faulting call.
+ */
+static volatile sig_atomic_t caught;
+
+static void segv(int sig)
+{
+    caught = 1;
+    siglongjmp(jb, 1);
+}
+
+/* lda $0, imm($31)  -> v0 = imm */
+static unsigned int lda_v0(int imm)
+{
+    return 0x201F0000u | (unsigned short)imm;
+}
+
+int main(void)
+{
+    struct sigaction sa;
+    int rc = 0;
+    unsigned char *m = mmap(NULL, 2 * PS, PROT_READ | PROT_WRITE | PROT_EXEC,
+                            MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    if (m == MAP_FAILED) {
+        perror("mmap");
+        return 2;
+    }
+
+    unsigned char *pa = m, *pb = m + PS;
+    unsigned int *entry = (unsigned int *)(pa + PS - 64);
+    unsigned int *tgt = (unsigned int *)(pb + 16);
+
+    entry[0] = lda_v0(1);
+    long disp = ((long)tgt - ((long)&entry[1] + 4)) / 4;
+    entry[1] = 0xC3E00000u | (unsigned int)(disp & 0x1FFFFF);  /* br $31,tgt */
+    tgt[0] = 0x6BFA8001u;                                      /* ret        */
+    __builtin___clear_cache((char *)m, (char *)m + 2 * PS);
+
+    long (*fn)(void) = (long (*)(void))entry;
+
+    for (int i = 0; i < 200000; i++) {
+        if (fn() != 1) {
+            printf("FAIL: phase 1 wrong result\n");
+            return 1;
+        }
+    }
+    printf("phase 1 ok (chained)\n");
+
+    memset(&sa, 0, sizeof(sa));
+    sa.sa_handler = segv;
+    sigemptyset(&sa.sa_mask);
+    if (sigaction(SIGSEGV, &sa, NULL) != 0) {
+        perror("sigaction");
+        return 2;
+    }
+    if (mprotect(pb, PS, PROT_NONE) != 0) {
+        perror("mprotect");
+        return 2;
+    }
+    if (sigsetjmp(jb, 1) == 0) {
+        fn();
+        printf("FAIL: phase 2 executed page B after mprotect(PROT_NONE)\n");
+        rc = 1;
+    } else if (!caught) {
+        printf("FAIL: phase 2 longjmp without entering the handler\n");
+        rc = 1;
+    } else {
+        printf("phase 2 ok (faulted)\n");
+    }
+
+    /* Phase 3: remap with different code, expect the new code to run. */
+    if (mprotect(pb, PS, PROT_READ | PROT_WRITE | PROT_EXEC) != 0) {
+        perror("mprotect back");
+        return 2;
+    }
+    tgt[0] = lda_v0(2);
+    tgt[1] = 0x6BFA8001u;
+    __builtin___clear_cache((char *)pb, (char *)pb + PS);
+
+    long r = fn();
+    if (r != 2) {
+        printf("FAIL: phase 3 returned %ld, expected 2 (stale chain)\n", r);
+        rc = 1;
+    } else {
+        printf("phase 3 ok (new code ran)\n");
+    }
+    return rc;
+}
-- 
2.54.0