Re: [PATCH v6 1/3] memory: drop guest writes to read-only ram device regions

[Yan Zhao <yan.y.zhao@intel.com>]

On Thu, May 28, 2020 at 07:10:46AM +0200, Paolo Bonzini wrote:
> On 28/05/20 06:35, Yan Zhao wrote:
> > On Tue, May 26, 2020 at 10:26:35AM +0100, Peter Maydell wrote:
> >> On Mon, 25 May 2020 at 11:20, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>> Not all of them, only those that need to return MEMTX_ERROR.  I would
> >>> like some guidance from Peter as to whether (or when) reads from ROMs
> >>> should return MEMTX_ERROR.  This way, we can use that information to
> >>> device  what the read-only ram-device regions should do.
> >>
> >> In general I think writes to ROMs (and indeed reads from ROMs) should
> >> not return MEMTX_ERROR. I think that in real hardware you could have
> >> a ROM that behaved either way; so our default behaviour should probably
> >> be to do what we've always done and not report a MEMTX_ERROR. (If we
> >> needed to I suppose we should implement a MEMTX_ERROR-reporting ROM,
> >> but to be honest there aren't really many real ROMs in systems these
> >> days: it's more often flash, whose response to writes is defined
> >> by the spec and is I think to ignore writes which aren't the
> >> magic "shift to program-the-flash-mode" sequence.)
> >>
> > then should I just drop the writes to read-only ram-device regions and
> > vfio regions without returning MEMTX_ERROR?
> > do you think it's good?
> 
> I am not really sure, I have to think more about it.  I think read-only
> RAMD regions are slightly different because the guest can expect "magic"
> behavior from RAMD regions (e.g. registers that trigger I/O on writes)
> that are simply not there for ROM.  So I'm still inclined to queue your
> v6 patch series.
> 
ok. thank you Paolo. :)