Re: [Qemu-devel] [edk2] [qemu PATCH] hw/i386/pc_sysfw: support more than one flash drive

[Paolo Bonzini]

Il 25/11/2013 20:45, Laszlo Ersek ha scritto:
> From looking at "hw/block/pflash_cfi01.c", it seems that the guest can
> interrogate the flash device about its size (nb_blocs is stored in
> cfi_table starting at 0x2D, and cfi_table can be queried by command 0x98
> in pflash_read()). So, if the guest cares, it can figure out that there
> are multiple devices backing the range. I think.

IIUC in the case of OVMF the guest "just knows" that there are multiple
devices backing the range.  Is that right?

But yes, I think that the guest can figure out that there are multiple
devices backing the range.  From reading the pflash code further:

* the pflash device doesn't care about the location where you write the
command (the exception is the "block erase" command)

* the pflash device only cares about the LSB you read from when you read
data from it

So you can use the last 256 bytes of the first flash (you know it ends
at 4GB) to check for the device (there's probably some suggested
protocol to do that, I don't know) and query its size.  Example with
"-qtest stdio":

writeb 0xffffff00 0x98
OK
readb 0xfffffff2d
OK 0x000000000000001f
readb 0xfffffff2e
OK 0x0000000000000000
readb 0xfffffff2f
OK 0x0000000000000010
readb 0xfffffff30
OK 0x0000000000000000
writeb 0xffffff00 0x98
OK

This means the device has 31+1 blocks each 4KB in size.  You can then
query the next device at 4GB-128KB-256, and so on.

Paolo