Re: Question about (and problem with) pflash data access

[Philippe Mathieu-Daudé]

On 2/13/20 3:39 PM, Peter Maydell wrote:
> On Thu, 13 Feb 2020 at 14:26, Guenter Roeck <address@hidden> wrote:
> > What really puzzles me is that there is no trace output for
> > flash data accesses (trace_pflash_data_read and trace_pflash_data_write),
> > meaning the actual flash data access must be handled elsewhere.
> > Can someone give me a hint where that might be ?

If you can share built kernel/dtb/rootfs for this machine I can have a 
look at it.

> > Clearly I am missing something about inner workings of qemu.

You can see all the pflash events using '-trace pflash*'.

> Probably the device is in 'romd' mode. A QEMU MemoryRegion
> can be:
>   * RAM (includes ROM for these purposes) -- backed by host
>     memory, reads and writes (if permitted) go straight to
>     the host memory via fastpath accesses

No tracing here.

>   * MMIO -- backed by a read and write accessor function,
>     all accesses go to these functions
>   * "ROM device" -- a mix of the above where there is a
>     backing bit of host memory but also accessor functions.
>     When the device is in "romd" mode, reads go direct to
>     host memory, and writes still go to the accessor function.
>     When the device is not in "romd" mode, reads also go
>     to the accessor function.
>
> We use this in the pflash devices to make the common case
> ("just read the flash") fast. When the guest makes a write
> to flash that puts it into programming mode, we call
> memory_region_rom_device_set_romd(..., false) so we can
> intercept reads and make them do the right thing for
> programming mode.
>
> thanks
> -- PMM