Re: [PATCH v1 1/3] softmmu/physmem: fallback to opening guest RAM file as readonly in a MAP_PRIVATE mapping

[Daniel P . Berrangé]

On Fri, Aug 11, 2023 at 09:00:54PM +0200, David Hildenbrand wrote:
> On 11.08.23 07:49, ThinerLogoer wrote:
> > At 2023-08-11 05:24:43, "Peter Xu" <peterx@redhat.com> wrote:
> > > On Fri, Aug 11, 2023 at 01:06:12AM +0800, ThinerLogoer wrote:
> > > > > I think we have the following options (there might be more)
> > > > > 
> > > > > 1) This patch.
> > > > > 
> > > > > 2) New flag for memory-backend-file. We already have "readonly" and
> > > > > "share=". I'm having a hard time coming up with a good name that 
> > > > > really
> > > > > describes the subtle difference.
> > > > > 
> > > > > 3) Glue behavior to the QEMU machine
> > > > > 
> > > > 
> > > > 4) '-deny-private-discard' argv, or environment variable, or both
> > > 
> > > I'd personally vote for (2).  How about "fdperm"?  To describe when we 
> > > want
> > > to use different rw permissions on the file (besides the access permission
> > > of the memory we already provided with "readonly"=XXX).  IIUC the only 
> > > sane
> > > value will be ro/rw/default, where "default" should just use the same rw
> > > permission as the memory ("readonly"=XXX).
> > > 
> > > Would that be relatively clean and also work in this use case?
> > > 
> > > (the other thing I'd wish we don't have that fallback is, as long as we
> > > have any of that "fallback" we'll need to be compatible with it since
> > > then, and for ever...)
> > 
> > If it must be (2), I would vote (2) + (4), with (4) adjust the default 
> > behavior of said `fdperm`.
> > Mainly because (private+discard) is itself not a good practice and (4) 
> > serves
> > as a good tool to help catch existing (private+discard) problems.
> 
> Instead of fdperm, maybe we could find a better name.
> 
> The man page of "open" says: The argument flags must include one of the
> following access modes: O_RDONLY, O_WRONLY, or O_RDWR.  These request
> opening the file read-only, write-only, or read/write, respectively.
> 
> So maybe something a bit more mouthful like "file-access-mode" would be
> better.

I don't think we should directly express the config in terms
of file-access-mode, as that's a low level impl detail. The
required file access mode is an artifact of the higher level
goal, or whether the RAM should be process private vs shared,
and whether we want QEMU to be able to create the backing
file or use pre-create one.

IOW, we should express whether or not we want QEMU to try to
pre-create the file or not.


With regards,
Daniel
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