Re: [Qemu-devel] [PATCH v6 1/7] linker-loader: Add new 'write pointer' command

[Laszlo Ersek]

On 02/15/17 15:13, Laszlo Ersek wrote:
> Commenting under Igor's reply for simplicity
> 
> On 02/15/17 11:57, Igor Mammedov wrote:
>> On Tue, 14 Feb 2017 22:15:43 -0800
>> address@hidden wrote:
>>
>>> From: Ben Warren <address@hidden>
>>>
>>> This is similar to the existing 'add pointer' functionality, but instead
>>> of instructing the guest (BIOS or UEFI) to patch memory, it instructs
>>> the guest to write the pointer back to QEMU via a writeable fw_cfg file.
>>>
>>> Signed-off-by: Ben Warren <address@hidden>
>>> ---
>>>  hw/acpi/bios-linker-loader.c         | 58 
>>> ++++++++++++++++++++++++++++++++++--
>>>  include/hw/acpi/bios-linker-loader.h |  6 ++++
>>>  2 files changed, 61 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/hw/acpi/bios-linker-loader.c b/hw/acpi/bios-linker-loader.c
>>> index d963ebe..5030cf1 100644
>>> --- a/hw/acpi/bios-linker-loader.c
>>> +++ b/hw/acpi/bios-linker-loader.c
>>> @@ -78,6 +78,19 @@ struct BiosLinkerLoaderEntry {
>>>              uint32_t length;
>>>          } cksum;
>>>  
>>> +        /*
>>> +         * COMMAND_WRITE_POINTER - write the fw_cfg file (originating from
>>> +         * @dest_file) at @wr_pointer.offset, by adding a pointer to the 
>>> table
>>> +         * originating from @src_file. 1,2,4 or 8 byte unsigned
>>> +         * addition is used depending on @wr_pointer.size.
>>> +         */
> 
> The words "adding" and "addition" are causing confusion here.
> 
> In all of the previous discussion, *addition* was out of scope from
> WRITE_POINTER. Again, the firmware is specifically not required to
> *read* any part of the fw_cfg blob identified by "dest_file".
> 
> WRITE_POINTER instructs the firmware to return the allocation address of
> the downloaded "src_file" to QEMU. Any necessary runtime subscripting
> within "src_file" is to be handled by QEMU code dynamically.
> 
> For example, consider that "src_file" has *several* fields that QEMU
> wants to massage; in that case, indexing within QEMU code with field
> offsets is simply unavoidable.
> 
> (1) So, the above looks correct, but please replace "adding" with
> "storing", and "unsigned addition" with "store".
> 
> Side point: the case for ADD_POINTER is different; there we patch
> several individual ACPI objects. The fact that I requested explicit
> addition within the ADDR method, as opposed to pre-setting VGIA to a
> nonzero offset, is an *incidental* limitation (coming from the OVMF ACPI
> SDT header probe suppressor), and we'll likely fix that up later, with
> ALLOCATE command hints or something like that. However, in
> WRITE_POINTER, asking for the exact allocation address of "src_file" is
> an *inherent* characteristic.
> 
> For reference, this is the command's description from the (not as yet
> posted) OVMF series:
> 
> // QemuLoaderCmdWritePointer: the bytes at
> // [PointerOffset..PointerOffset+PointerSize) in the writeable fw_cfg
> // file PointerFile are to receive the absolute address of PointeeFile,
> // as allocated and downloaded by the firmware. Store the base address
> // of where PointeeFile's contents have been placed (when
> // QemuLoaderCmdAllocate has been executed for PointeeFile) to this
> // portion of PointerFile.
> //
> // This command is similar to QemuLoaderCmdAddPointer; the difference is
> // that the "pointer to patch" does not exist in guest-physical address
> // space, only in "fw_cfg file space". In addition, the "pointer to
> // patch" is not initialized by QEMU with a possibly nonzero offset
> // value: the base address of the memory allocated for downloading
> // PointeeFile shall not increment the pointer, but overwrite it.
> 
> In the last SeaBIOS patch series, namely
> 
> [SeaBIOS] [PATCH v3 2/2] QEMU fw_cfg: Add command to write back address
>                          of file
> 
> function romfile_loader_write_pointer() implemented just that plain
> store (not an addition), and that was exactly right.
> 
> Continuing:
> 
>>> +        struct {
>>> +            char dest_file[BIOS_LINKER_LOADER_FILESZ];
>>> +            char src_file[BIOS_LINKER_LOADER_FILESZ];
>>> +            uint32_t offset;
>>> +            uint8_t size;
>>> +        } wr_pointer;
>>> +
>>>          /* padding */
>>>          char pad[124];
>>>      };
>>> @@ -85,9 +98,10 @@ struct BiosLinkerLoaderEntry {
>>>  typedef struct BiosLinkerLoaderEntry BiosLinkerLoaderEntry;
>>>  
>>>  enum {
>>> -    BIOS_LINKER_LOADER_COMMAND_ALLOCATE     = 0x1,
>>> -    BIOS_LINKER_LOADER_COMMAND_ADD_POINTER  = 0x2,
>>> -    BIOS_LINKER_LOADER_COMMAND_ADD_CHECKSUM = 0x3,
>>> +    BIOS_LINKER_LOADER_COMMAND_ALLOCATE          = 0x1,
>>> +    BIOS_LINKER_LOADER_COMMAND_ADD_POINTER       = 0x2,
>>> +    BIOS_LINKER_LOADER_COMMAND_ADD_CHECKSUM      = 0x3,
>>> +    BIOS_LINKER_LOADER_COMMAND_WRITE_POINTER     = 0x4,
>>>  };
>>>  
>>>  enum {
>>> @@ -278,3 +292,41 @@ void bios_linker_loader_add_pointer(BIOSLinker *linker,
>>>  
>>>      g_array_append_vals(linker->cmd_blob, &entry, sizeof entry);
>>>  }
>>> +
>>> +/*
>>> + * bios_linker_loader_write_pointer: ask guest to write a pointer to the
>>> + * source file into the destination file, and write it back to QEMU via
>>> + * fw_cfg DMA.
>>> + *
>>> + * @linker: linker object instance
>>> + * @dest_file: destination file that must be written
>>> + * @dst_patched_offset: location within destination file blob to be patched
>>> + *                      with the pointer to @src_file, in bytes
>>> + * @dst_patched_offset_size: size of the pointer to be patched
>>> + *                      at @dst_patched_offset in @dest_file blob, in bytes
>>> + * @src_file: source file who's address must be taken
>>> + */
>>> +void bios_linker_loader_write_pointer(BIOSLinker *linker,
>>> +                                    const char *dest_file,
>>> +                                    uint32_t dst_patched_offset,
>>> +                                    uint8_t dst_patched_size,
>>> +                                    const char *src_file)
>> API is missing "src_offset" even though it's not used in this series,
>> a patch on top to fix it up is ok for me as far as Seabios/OVMF
>> counterpart can handle src_offset correctly from starters.
> 
> According to the above, it is the right thing not to add "src_offset"
> here. The documentation on the command is slightly incorrect (and causes
> confusion), but the helper function's signature and comments are okay.
> 
>>
>>> +{
>>> +    BiosLinkerLoaderEntry entry;
>>> +    const BiosLinkerFileEntry *source_file =
>>> +        bios_linker_find_file(linker, src_file);
>>> +
>>> +    assert(source_file);
> 
> I wish we kept the following asserts from bios_linker_loader_add_pointer():
> 
>     assert(dst_patched_offset < dst_file->blob->len);
>     assert(dst_patched_offset + dst_patched_size <= dst_file->blob->len);
> 
> Namely, just because the dst_file is never supposed to be downloaded by
> the firmware, it still remains a requirement that the "dst file offset
> range" that is to be rewritten *do fall* within the dst file.
> 
> Nonetheless, this is not critical. (OVMF at least verifies it anyway.)

Update: here I missed for a moment that bios_linker_find_file() would
not be able to locate dst_file. The reason for that is that we
(correctly!) never produce an ALLOCATE command for dst_file, hence we
never add it to "linker->file_list" either.

Therefore, please ignore this comment about the assert()s. My only
comment for this patch remains the docs update, as described in (1).

Thank you,
Laszlo

> 
> Summary (from my side anyway): I feel that the documentation of the new
> command is very important. Please fix it up as suggested under (1), in
> v7. Regarding the asserts, I'll let you decide.
> 
> With the documentation fixed up:
> 
> Reviewed-by: Laszlo Ersek <address@hidden>
> 
> (If you don't wish to post a v7, I'm also completely fine if Michael or
> someone else fixes up the docs as proposed in (1), before committing the
> patch.)
> 
> Thanks!
> Laszlo
> 
>>> +    memset(&entry, 0, sizeof entry);
>>> +    strncpy(entry.wr_pointer.dest_file, dest_file,
>>> +            sizeof entry.wr_pointer.dest_file - 1);
>>> +    strncpy(entry.wr_pointer.src_file, src_file,
>>> +            sizeof entry.wr_pointer.src_file - 1);
>>> +    entry.command = cpu_to_le32(BIOS_LINKER_LOADER_COMMAND_WRITE_POINTER);
>>> +    entry.wr_pointer.offset = cpu_to_le32(dst_patched_offset);
>>> +    entry.wr_pointer.size = dst_patched_size;
>>> +    assert(dst_patched_size == 1 || dst_patched_size == 2 ||
>>> +           dst_patched_size == 4 || dst_patched_size == 8);
>>> +
>>> +    g_array_append_vals(linker->cmd_blob, &entry, sizeof entry);
>>> +}
>>> diff --git a/include/hw/acpi/bios-linker-loader.h 
>>> b/include/hw/acpi/bios-linker-loader.h
>>> index fa1e5d1..f9ba5d6 100644
>>> --- a/include/hw/acpi/bios-linker-loader.h
>>> +++ b/include/hw/acpi/bios-linker-loader.h
>>> @@ -26,5 +26,11 @@ void bios_linker_loader_add_pointer(BIOSLinker *linker,
>>>                                      const char *src_file,
>>>                                      uint32_t src_offset);
>>>  
>>> +void bios_linker_loader_write_pointer(BIOSLinker *linker,
>>> +                                      const char *dest_file,
>>> +                                      uint32_t dst_patched_offset,
>>> +                                      uint8_t dst_patched_size,
>>> +                                      const char *src_file);
>>> +
>>>  void bios_linker_loader_cleanup(BIOSLinker *linker);
>>>  #endif
>>
>