hw/core/loader: Add new function rom_ptr_for_as()

For accesses to rom blob data before or during reset, we have a
function rom_ptr() which looks for a rom blob that would be loaded to
the specified address, and returns a pointer into the rom blob data
corresponding to that address.  This allows board or CPU code to say
"what is the data that is going to be loaded to this address?".

However, this function does not take account of memory region
aliases.  If for instance a machine model has RAM at address
0x0000_0000 which is aliased to also appear at 0x1000_0000, a
rom_ptr() query for address 0x0000_0000 will only return a match if
the guest image provided by the user was loaded at 0x0000_0000 and
not if it was loaded at 0x1000_0000, even though they are the same
RAM and a run-time guest CPU read of 0x0000_0000 will read the data
loaded to 0x1000_0000.

Provide a new function rom_ptr_for_as() which takes an AddressSpace
argument, so that it can check whether the MemoryRegion corresponding
to the address is also mapped anywhere else in the AddressSpace and
look for rom blobs that loaded to that alias.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210318174823.18066-5-peter.maydell@linaro.org

2 files changed, 106 insertions, 0 deletions

diff --git a/hw/core/loader.c b/hw/core/loader.c
index 9feca32de9..d3e5f3b423 100644
--- a/hw/core/loader.c
+++ b/hw/core/loader.c
@@ -1383,6 +1383,81 @@ void *rom_ptr(hwaddr addr, size_t size)
     return rom->data + (addr - rom->addr);
 }
 
+typedef struct FindRomCBData {
+    size_t size; /* Amount of data we want from ROM, in bytes */
+    MemoryRegion *mr; /* MR at the unaliased guest addr */
+    hwaddr xlat; /* Offset of addr within mr */
+    void *rom; /* Output: rom data pointer, if found */
+} FindRomCBData;
+
+static bool find_rom_cb(Int128 start, Int128 len, const MemoryRegion *mr,
+                        hwaddr offset_in_region, void *opaque)
+{
+    FindRomCBData *cbdata = opaque;
+    hwaddr alias_addr;
+
+    if (mr != cbdata->mr) {
+        return false;
+    }
+
+    alias_addr = int128_get64(start) + cbdata->xlat - offset_in_region;
+    cbdata->rom = rom_ptr(alias_addr, cbdata->size);
+    if (!cbdata->rom) {
+        return false;
+    }
+    /* Found a match, stop iterating */
+    return true;
+}
+
+void *rom_ptr_for_as(AddressSpace *as, hwaddr addr, size_t size)
+{
+    /*
+     * Find any ROM data for the given guest address range.  If there
+     * is a ROM blob then return a pointer to the host memory
+     * corresponding to 'addr'; otherwise return NULL.
+     *
+     * We look not only for ROM blobs that were loaded directly to
+     * addr, but also for ROM blobs that were loaded to aliases of
+     * that memory at other addresses within the AddressSpace.
+     *
+     * Note that we do not check @as against the 'as' member in the
+     * 'struct Rom' returned by rom_ptr(). The Rom::as is the
+     * AddressSpace which the rom blob should be written to, whereas
+     * our @as argument is the AddressSpace which we are (effectively)
+     * reading from, and the same underlying RAM will often be visible
+     * in multiple AddressSpaces. (A common example is a ROM blob
+     * written to the 'system' address space but then read back via a
+     * CPU's cpu->as pointer.) This does mean we might potentially
+     * return a false-positive match if a ROM blob was loaded into an
+     * AS which is entirely separate and distinct from the one we're
+     * querying, but this issue exists also for rom_ptr() and hasn't
+     * caused any problems in practice.
+     */
+    FlatView *fv;
+    void *rom;
+    hwaddr len_unused;
+    FindRomCBData cbdata = {};
+
+    /* Easy case: there's data at the actual address */
+    rom = rom_ptr(addr, size);
+    if (rom) {
+        return rom;
+    }
+
+    RCU_READ_LOCK_GUARD();
+
+    fv = address_space_to_flatview(as);
+    cbdata.mr = flatview_translate(fv, addr, &cbdata.xlat, &len_unused,
+                                   false, MEMTXATTRS_UNSPECIFIED);
+    if (!cbdata.mr) {
+        /* Nothing at this address, so there can't be any aliasing */
+        return NULL;
+    }
+    cbdata.size = size;
+    flatview_for_each_range(fv, find_rom_cb, &cbdata);
+    return cbdata.rom;
+}
+
 void hmp_info_roms(Monitor *mon, const QDict *qdict)
 {
     Rom *rom;
diff --git a/include/hw/loader.h b/include/hw/loader.h
index a9eeea3952..cbfc184873 100644
--- a/include/hw/loader.h
+++ b/include/hw/loader.h
@@ -290,6 +290,37 @@ void rom_transaction_end(bool commit);
 
 int rom_copy(uint8_t *dest, hwaddr addr, size_t size);
 void *rom_ptr(hwaddr addr, size_t size);
+/**
+ * rom_ptr_for_as: Return a pointer to ROM blob data for the address
+ * @as: AddressSpace to look for the ROM blob in
+ * @addr: Address within @as
+ * @size: size of data required in bytes
+ *
+ * Returns: pointer into the data which backs the matching ROM blob,
+ * or NULL if no blob covers the address range.
+ *
+ * This function looks for a ROM blob which covers the specified range
+ * of bytes of length @size starting at @addr within the address space
+ * @as. This is useful for code which runs as part of board
+ * initialization or CPU reset which wants to read data that is part
+ * of a user-supplied guest image or other guest memory contents, but
+ * which runs before the ROM loader's reset function has copied the
+ * blobs into guest memory.
+ *
+ * rom_ptr_for_as() will look not just for blobs loaded directly to
+ * the specified address, but also for blobs which were loaded to an
+ * alias of the region at a different location in the AddressSpace.
+ * In other words, if a machine model has RAM at address 0x0000_0000
+ * which is aliased to also appear at 0x1000_0000, rom_ptr_for_as()
+ * will return the correct data whether the guest image was linked and
+ * loaded at 0x0000_0000 or 0x1000_0000.  Contrast rom_ptr(), which
+ * will only return data if the image load address is an exact match
+ * with the queried address.
+ *
+ * New code should prefer to use rom_ptr_for_as() instead of
+ * rom_ptr().
+ */
+void *rom_ptr_for_as(AddressSpace *as, hwaddr addr, size_t size);
 void hmp_info_roms(Monitor *mon, const QDict *qdict);
 
 #define rom_add_file_fixed(_f, _a, _i)          \