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

[Ben Warren]

> On Feb 15, 2017, at 9:43 AM, Igor Mammedov <address@hidden> wrote:
> 
> On Wed, 15 Feb 2017 18:39:06 +0200
> "Michael S. Tsirkin" <address@hidden <mailto:address@hidden>> wrote:
> 
>> On Wed, Feb 15, 2017 at 04:56:02PM +0100, Igor Mammedov wrote:
>>> On Wed, 15 Feb 2017 17:30:00 +0200
>>> "Michael S. Tsirkin" <address@hidden> wrote:
>>> 
>>>> On Wed, Feb 15, 2017 at 04:22:25PM +0100, Igor Mammedov wrote:  
>>>>> On Wed, 15 Feb 2017 15:13:20 +0100
>>>>> Laszlo Ersek <address@hidden> 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.    
>>>>> what I don't like here is that this indexing would be rather fragile
>>>>> and has to be done in different parts of QEMU /device, AML/.
>>>>> 
>>>>> I'd prefer this helper function to have the same @src_offset
>>>>> behavior as ADD_POINTER where patched address could point to
>>>>> any part of src_file i.e. not just beginning.    
>>>> 
>>>> 
>>>> 
>>>>        /*
>>>>         * COMMAND_ADD_POINTER - patch the table (originating from
>>>>         * @dest_file) at @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 @pointer.size.
>>>>         */
>>>> 
>>>> so the way ADD works is
>>>>    read at offset
>>>>    add table address
>>>>    write result at offset
>>>> 
>>>> in other words it is always beginning of table that is added.  
>>> more exactly it's, read at 
>>>  src_offset = *(dst_blob_ptr+dst_offset)
>>>  *(dst_blob+dst_offset) = src_blob_ptr + src_offset
>>> 
>>>> Would the following be acceptable?
>>>> 
>>>> 
>>>>         * COMMAND_WRITE_POINTER - update the fw_cfg file (originating from
>>>>         * @dest_file) at @wr_pointer.offset, by writing a pointer to the 
>>>> table
>>>>         * originating from @src_file. 1,2,4 or 8 byte unsigned value
>>>>         * is written depending on @wr_pointer.size.  
>>> it looses 'adding' part of ADD_POINTER command which handles src_offset,
>>> however implementing adding part looks a bit complicated
>>> as patched blob (dst) is not in guest memory but in QEMU and
>>> on reset *(dst_blob+dst_offset) should be reset to src_offset.
>>> Considering dst file could be device specific memory (field/blob/whatever)
>>> it could be hard to track/notice proper reset behavior.
>>> 
>>> So now I'm not sure if src_offset is worth adding.  
>> 
>> Right. Let's just do this math in QEMU if we have to.
> Math complicates QEMU code though and not only QMEMU but AML code as well.
> Considering that we are adding a new command and don't have to keep
> any sort of compatibility we can pass src_offset as part
> of command instead of hiding it inside of dst_file.
> Something like this:
> 
>        /*
>         * COMMAND_WRITE_POINTER - write the fw_cfg file (originating from
>         * @dest_file) at @wr_pointer.offset, by writing a pointer to 
> @src_offset
>         * within the table originating from @src_file. 1,2,4 or 8 byte 
> unsigned
>         * addition is used depending on @wr_pointer.size.
>         */
>        struct {
>             char dest_file[BIOS_LINKER_LOADER_FILESZ];
>             char src_file[BIOS_LINKER_LOADER_FILESZ];
> -            uint32_t offset;
> +            uint32_t dst_offset;
> +            uint32_t src_offset;
>             uint8_t size;
>        } wr_pointer;
> 
OK, this is easy enough to do and maybe we’ll have a use case in the future.  
I’ll make this change in v7

>> 
>>>> 
>>>> 
>>>>> 
>>>>> 
>>>>>> (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.)
>>>>>> 
>>>>>> 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

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