Re: [PATCH v6 1/4] memory: prevent dma-reentracy issues

[Alexander Bulekov]

On 230310 0802, Alexander Bulekov wrote:
> On 230310 1245, Peter Maydell wrote:
> > On Fri, 10 Mar 2023 at 12:32, Alexander Bulekov <alxndr@bu.edu> wrote:
> > > This MR seems to be "lsi-ram".
> > >
> > > From hw/scsi/lsi53c895a.c:
> > >
> > > memory_region_init_io(&s->ram_io, OBJECT(s), &lsi_ram_ops, s,
> > >         "lsi-ram", 0x2000);
> > >
> > > So the LSI device is reading an LSI "Script" from its own IO region.. In
> > > this particular case, I think there was no reason to use
> > > memory_region_init_io rather than memory_region_init_ram, but this makes
> > > me worried that there are other devices that use something like this.
> > 
> > This particular device predates the entire MemoryRegion set of
> > abstractions, so it might have seemed easier at the time.
> > The endianness handling of the current code is also a bit
> > confusing and might make it tricky to convert to a RAM MR.
> 
> With my trivial mr_io - > mr_ram conversion, I no longer hit the
> re-entrancy tracepoint, and my livecd boots, but it's probably not
> exhaustively using the script functionality.. 
> 
> Does the endianness actually cause a problem? As long as all accesses
> (CPU -> LSI_RAM and LSI -> LSI_RAM) occur through the address_space API,
> are we safe?
> -Alex

Or maybe a rom_device with memory_region_rom_device_set_romd(romd_mode =
false) is better here?