Re: [Qemu-devel] [PATCH v4 1/9] ACPI: Add a function for building named qword entries

[Ben Warren]

> On Jan 27, 2017, at 8:12 AM, Laszlo Ersek <address@hidden> wrote:
> 
> On 01/27/17 16:43, Kevin O'Connor wrote:
>> On Fri, Jan 27, 2017 at 03:46:33PM +0100, Laszlo Ersek wrote:
>>> On 01/27/17 15:18, Kevin O'Connor wrote:
>>>> If an offset is going to be added, shouldn't both a source offset and
>>>> destination offset be used?
>>>> 
>>>>        /*
>>>>         * COMMAND_WRITE_POINTER - update a writeable file named
>>>>         * @pointer.dest_file at @pointer.dest_offset, by writing pointer
>>>>         * plus @pointer.src_offset to the blob originating from
>>>>         * @src_file. 1,2,4 or 8 byte unsigned write is used depending
>>>>         * on @pointer.size.
>>>>         */
>>>>        struct {
>>>>            char dest_file[BIOS_LINKER_LOADER_FILESZ];
>>>>            char src_file[BIOS_LINKER_LOADER_FILESZ];
>>>>            uint32_t src_offset, dest_offset;
>>>>            uint8_t size;
>>>>        } pointer;
>>>> 
>>>> I doubt the offsets or size is really all that important though.
>>> 
>>> The offset into the fw_cfg file that receives the allocation address is
>>> important, that allows the same file to receive several different
>>> addresses (for different downloaded blobs), at different offsets.
>>> 
>>> OTOH, asking the firmware to add a constant to the address value before
>>> writing it to the fw_cfg file is not necessary, in my opinion. The blob
>>> that the firmware allocated and downloaded originates from QEMU to begin
>>> with, so QEMU knows its internal structure.
>> 
>> I guess I'm missing why QEMU would want to use the same writable file
>> for multiple pointers
> 
> I know no specific reason; I just thought this possible generalization
> was one of the advantages in Michael's suggestion.
> 
>> as well as why it would want support for
>> pointers smaller than 8 bytes in size.
> 
> Hm, right, good point.
> 
>> If it's because it may be
>> easier to support an internal QEMU blob of a particular format, then
>> adding a src_offset would facilitate that.
>> 
>> However, if it was done so that WRITE_POINTER mimicks ADD_POINTER then
>> that's fine too.
> 
> That might be the main reason I guess; reading back a bit, Michael wrote
> "... a variant of ADD_POINTER".
> 
>> I'm okay with either format.
> 
> I'd say let's go ahead with Michael's proposal then. Ben, are you okay
> with that?
> 
Yes, this seems reasonable.  If I’m interpreting this properly, it’s like 
ADD_POINTER, except that we extend the write path back to QEMU over DMA.  Is 
that about right?

> Thanks!
> Laszlo
—Ben

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