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

1 files changed, 75 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;