[PATCH v2] 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 as well.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---

v1: https://lists.gnu.org/archive/html/qemu-devel/2022-07/msg00112.html
v1 -> v2: Fix PAGE_PASSTHROUGH value; make checks consistent with
          page_set_flags() (Laurent).

 include/exec/cpu-all.h |  6 ++++++
 linux-user/mmap.c      | 25 +++++++++++++++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index f5bda2c3ca..2d29ba13c0 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 4e7a6be6ee..79928e3ae5 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -424,7 +424,8 @@ 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();
@@ -537,6 +538,8 @@ 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;
@@ -619,6 +622,8 @@ 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:
@@ -626,7 +631,18 @@ 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)) {
@@ -845,7 +861,7 @@ 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_ANON | PAGE_PASSTHROUGH))) {
             return false;
         }
     }
@@ -888,7 +904,8 @@ abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice)
      *
      * 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.
+     * anonymous mappings or 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 & MADV_DONTNEED) &&
-- 
2.35.3