[PATCH v3 4/5] linux-user: Passthrough MADV_DONTNEED for certain file mappings

[Ilya Leoshkevich <iii@linux.ibm.com>]

This is a follow-up for commit 892a4f6a750a ("linux-user: Add partial
support for MADV_DONTNEED"), which added passthrough for anonymous
mappings. File mappings can be handled in a similar manner.

In order to do that, mark pages, for which mmap() was passed through,
with PAGE_PASSTHROUGH, and then allow madvise() passthrough for these
pages. Drop the explicit PAGE_ANON check, since anonymous mappings are
expected to have PAGE_PASSTHROUGH anyway.

Add PAGE_PASSTHROUGH to PAGE_STICKY in order to keep it on mprotect().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20220725125043.43048-1-iii@linux.ibm.com>
---
 accel/tcg/translate-all.c |  2 +-
 include/exec/cpu-all.h    |  6 ++++++
 linux-user/mmap.c         | 27 ++++++++++++++++++++++-----
 3 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
index b83161a081..a47cf38e38 100644
--- a/accel/tcg/translate-all.c
+++ b/accel/tcg/translate-all.c
@@ -2263,7 +2263,7 @@ int page_get_flags(target_ulong address)
 #ifndef PAGE_TARGET_STICKY
 #define PAGE_TARGET_STICKY  0
 #endif
-#define PAGE_STICKY  (PAGE_ANON | PAGE_TARGET_STICKY)
+#define PAGE_STICKY  (PAGE_ANON | PAGE_PASSTHROUGH | PAGE_TARGET_STICKY)
 
 /* Modify the flags of a page and invalidate the code if necessary.
    The flag PAGE_WRITE_ORG is positioned automatically depending
diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index 491629b9ba..16b7df41bf 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -262,6 +262,12 @@ extern const TargetPageBits target_page;
 #define PAGE_TARGET_1  0x0200
 #define PAGE_TARGET_2  0x0400
 
+/*
+ * For linux-user, indicates that the page is mapped with the same semantics
+ * in both guest and host.
+ */
+#define PAGE_PASSTHROUGH 0x0800
+
 #if defined(CONFIG_USER_ONLY)
 void page_dump(FILE *f);
 
diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index a5f1ab129c..3a0f67619a 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -425,7 +425,8 @@ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size, abi_ulong align)
 abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot,
                      int flags, int fd, abi_ulong offset)
 {
-    abi_ulong ret, end, real_start, real_end, retaddr, host_offset, host_len;
+    abi_ulong ret, end, real_start, real_end, retaddr, host_offset, host_len,
+              passthrough_start = -1, passthrough_end = -1;
     int page_flags, host_prot;
 
     mmap_lock();
@@ -538,6 +539,8 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot,
             host_start += offset - host_offset;
         }
         start = h2g(host_start);
+        passthrough_start = start;
+        passthrough_end = start + len;
     } else {
         if (start & ~TARGET_PAGE_MASK) {
             errno = EINVAL;
@@ -620,6 +623,8 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot,
                      host_prot, flags, fd, offset1);
             if (p == MAP_FAILED)
                 goto fail;
+            passthrough_start = real_start;
+            passthrough_end = real_end;
         }
     }
  the_end1:
@@ -627,7 +632,18 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot,
         page_flags |= PAGE_ANON;
     }
     page_flags |= PAGE_RESET;
-    page_set_flags(start, start + len, page_flags);
+    if (passthrough_start == passthrough_end) {
+        page_set_flags(start, start + len, page_flags);
+    } else {
+        if (start < passthrough_start) {
+            page_set_flags(start, passthrough_start, page_flags);
+        }
+        page_set_flags(passthrough_start, passthrough_end,
+                       page_flags | PAGE_PASSTHROUGH);
+        if (passthrough_end < start + len) {
+            page_set_flags(passthrough_end, start + len, page_flags);
+        }
+    }
  the_end:
     trace_target_mmap_complete(start);
     if (qemu_loglevel_mask(CPU_LOG_PAGE)) {
@@ -846,7 +862,7 @@ static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end)
     }
 
     for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) {
-        if (!(page_get_flags(addr) & PAGE_ANON)) {
+        if (!(page_get_flags(addr) & PAGE_PASSTHROUGH)) {
             return false;
         }
     }
@@ -888,8 +904,9 @@ abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice)
      * This is a hint, so ignoring and returning success is ok.
      *
      * This breaks MADV_DONTNEED, completely implementing which is quite
-     * complicated. However, there is one low-hanging fruit: host-page-aligned
-     * anonymous mappings. In this case passthrough is safe, so do it.
+     * complicated. However, there is one low-hanging fruit: mappings that are
+     * known to have the same semantics in the host and the guest. In this case
+     * passthrough is safe, so do it.
      */
     mmap_lock();
     if (advice == TARGET_MADV_DONTNEED &&
-- 
2.37.2