Features/PC System Flash

This is a proposal for a simple flash emulation on QEMU/KVM.

Command line interface

To enable saving of the flash contents, a -flash a command line parameter would be added. This parameter normally will override the -bios file usage.

-flash path     path to filename for saving flash contents

If the flash path does not exist, then the -bios file will be read to initialize the flash contents. If the flash path does exist, then it will be used instead of the -bios file contents.

VM hardware interface

Hardware resources

The flash will be available as memory mapped io just below 0x100000000 (4GB). For example, if the -flash file was 0x100000 (1MB) in size, then the flash will be available in memory space from 0xfff00000 through 0xffffffff.

Normal reads

The flash content can be read in 1, 2, 4 or 8 byte sizes like the current read-only bios rom. The read accesses are not required to be aligned.

Programming interface

Aside from normal flash read operations, the flash can be communicated with via special sequences of memory read and write operations.

This interface supports 32-bit aligned 32-bit data operations as well as 64-bit aligned 64-bit operations.

In the following examples, the address 0xfffffffc is used for 32-bit examples, and 0xfffffff8 is used for 64-bit examples. However, any properly aligned address can be used from 0x100000000-sizeof(bios.bin) up through 0xfffffff8 (for 64-bit operations) or 0xfffffffc (for 32-bit operations).

Only 1 flash communication operation can occur at one time. If any flash write occurs in another area of the flash, the the previous operation will be cancelled.

Entering flash communication mode

32-bit access:

write32(addr=0xfffffffc, data=0x5a5a5a5a)
read32(addr=0xfffffffc) => returns 0xa5a5a5a5

64-bit access:

write64(addr=0xfffffff8, data=0x5a5a5a5a5a5a5a5a)
read64(addr=0xfffffff8) => returns 0xa5a5a5a5a5a5a5a5

Program flash

• The flash must be in flash communication mode

32-bit access:

write32(addr=0xfffffffc, data=0x0)
read32(addr=0xfffffffc) => returns current flash contents
write32(addr=0xfffffffc, data=new flash contents)
read32(addr=0xfffffffc) => returns new flash contents
read32(addr=0xfffffffc) => returns new flash contents ^ 0xffffffff

64-bit access:

write64(addr=0xfffffff8, data=0x0)
read64(addr=0xfffffff8) => returns current flash contents
write64(addr=0xfffffff8, data=new flash contents)
read64(addr=0xfffffff8) => returns new flash contents
read64(addr=0xfffffff8) => returns new flash contents ^ 0xffffffffffffffff

• Following a successful program operation, the flash will immediately return to normal read mode.

Flash size read

• The flash must be in flash communication mode

32-bit access:

write32(addr=0xfffffffc, data=0x1)
read32(addr=0xfffffffc) => returns the size of the flash device
read32(addr=0xfffffffc) => returns the flash size ^ 0xffffffff

64-bit access:

write64(addr=0xfffffff8, data=0x1)
read64(addr=0xfffffff8) => returns the size of the flash device
read64(addr=0xfffffff8) => returns the flash size ^ 0xffffffffffffffff

• Following a flash size read operation, the flash will immediately return to normal read mode.

Interruptions

• If the sequences above are not followed exactly, then the flash device should cancel the operation and immediately enter read mode.
• If a non-aligned write operation or a 1 or 2 byte write occurs to the flash while a flash operation is in progress, then the current operation is immediately cancelled.
• If a properly aligned write operation occurs to an different address or of a different size while another operation is in progress, then the current operation is immediately cancelled. If the write was the proper value to begin entering flash communication mode, then it will be accepted, and the sequence can proceed as a new operation.

Examples

• 32-bit flash write example at address 0xffff0000. Data: old=0x76543210, new=0xfedcba98

write32(addr=0xffff0000, data=0x5a5a5a5a)
read32(addr=0xffff0000) => returns 0xa5a5a5a5
write32(addr=0xffff0000, data=0x0)
read32(addr=0xffff0000) => returns 0x76543210
write32(addr=0xffff0000, data=0xfedcba98)
read32(addr=0xffff0000) => returns 0xfedcba98
read32(addr=0xffff0000) => returns 0x01234567

• 64-bit read flash size example at address 0xffff0000. Flash size=0x100000

write64(addr=0xffff0000, data=0x5a5a5a5a5a5a5a5a)
read64(addr=0xffff0000) => returns 0xa5a5a5a5a5a5a5a5
write64(addr=0xffff0000, data=0x1)
read64(addr=0xffff0000) => returns 0x0000000000100000
read64(addr=0xffff0000) => returns 0xffffffffffefffff

Why this complicated?

Programming flash in this proposal is simplistic by real hardware standards. Yet, for a VM we could easily make the flash region use a simple read/write interface.

The downside to this would be that buggy code which inadvertently writes to the flash region might very easily corrupt the flash image of the VM. With a little complexity required to write the flash, we guard against this issue in most cases.