Re: [Qemu-devel] [RFC PATCH] QEMU may write to system_memory before guest starts

[Peter Maydell]

On Tue, 19 Mar 2019 at 09:40, Dr. David Alan Gilbert
<address@hidden> wrote:
> I thought that ROMs would either:
>    a) Be mapped shared from a file but then read-only and unwritten

I don't think we support this at all, do we?

> or
>    b) Be written to during boot - but this wouldn't be main memory, so
> wouldn't be affected by your shared flag.

The rom code in hw/core/loader.c has two basic cases:
 (1) file is being loaded to something that is really a pure ROM:
in this case, on first reset we will write it to the backing RAMBlock
but we will not bother to do so in future
 (2) file is being loaded to something that is not a pure ROM:
this could be either real RAM (eg if a file is loaded via -kernel
or the 'generic loader' device), or a flash device, for instance.
In this case we will reload the backing RAMBlock with the file
contents on every reset, because the guest might have changed its
contents.
(There's also "file is provided via the fw_cfg device" but that's
not relevant here.)

I didn't think migration distinguished between "main memory"
and any other kind of RAMBlock-backed memory ?

thanks
-- PMM