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Re: [Qemu-devel] [PATCH v10 6/6] tpm: add ACPI memory clear interface


From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v10 6/6] tpm: add ACPI memory clear interface
Date: Wed, 19 Sep 2018 11:58:22 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

* Marc-André Lureau (address@hidden) wrote:
> Hi
> 
> On Tue, Sep 18, 2018 at 7:49 PM Dr. David Alan Gilbert
> <address@hidden> wrote:
> >
> > * Marc-André Lureau (address@hidden) wrote:
> > > Hi
> > >
> > > On Tue, Sep 11, 2018 at 6:19 PM Laszlo Ersek <address@hidden> wrote:
> > > >
> > > > +Alex, due to mention of 21e00fa55f3fd
> > > >
> > > > On 09/10/18 15:03, Marc-André Lureau wrote:
> > > > > Hi
> > > > >
> > > > > On Mon, Sep 10, 2018 at 2:44 PM Dr. David Alan Gilbert
> > > > > <address@hidden> wrote:
> > > > >> (I didn't know about guest_phys_block* and would have probably just 
> > > > >> used
> > > > >> qemu_ram_foreach_block )
> > > > >>
> > > > >
> > > > > guest_phys_block*() seems to fit, as it lists only the blocks actually
> > > > > used, and already skip the device RAM.
> > > > >
> > > > > Laszlo, you wrote the functions
> > > > > (https://git.qemu.org/?p=qemu.git;a=commit;h=c5d7f60f0614250bd925071e25220ce5958f75d0),
> > > > > do you think it's appropriate to list the memory to clear, or we
> > > > > should rather use qemu_ram_foreach_block() ?
> > > >
> > > > Originally, I would have said, "use either, doesn't matter". Namely,
> > > > when I introduced the guest_phys_block*() functions, the original
> > > > purpose was not related to RAM *contents*, but to RAM *addresses*
> > > > (GPAs). This is evident if you look at the direct child commit of
> > > > c5d7f60f0614, namely 56c4bfb3f07f, which put GuestPhysBlockList to use.
> > > > And, for your use case (= wiping RAM), GPAs don't matter, only contents
> > > > matter.
> > > >
> > > > However, with the commits I mentioned previously, namely e4dc3f5909ab9
> > > > and 21e00fa55f3fd, we now filter out some RAM blocks from the dumping
> > > > based on contents / backing as well. I think? So I believe we should
> > > > honor that for the wiping to. I guess I'd (vaguely) suggest using
> > > > guest_phys_block*().
> > > >
> > > > (And then, as Dave suggests, maybe extend the filter to consider pmem
> > > > too, separately.)
> > >
> > > I looked a bit into skipping pmem memory. The issue is that RamBlock
> > > and MemoryRegion have no idea they are actually used for nvram (you
> > > could rely on hostmem.pmem flag, but this is optional), and I don't
> > > see a clear way to figure this out.
> >
> > I think the pmem flag is what we should use; the problem though is we
> 
> That would be much simpler. But What if you setup a nvdimm backend by
> a non-pmem memory? It will always be cleared? What about platforms
> that do not support libpmem?

Right, that's basically the problem I say below, the difference between
(a) and (b).

> > have two different pieces of semantics:
> >     a) PMEM - needs special flush instruction/calls
> >     b) PMEM - my data is persistent, please don't clear me
> >
> > Do those always go together?
> >
> > (Copying in Junyan He who added the RAM_PMEM flag)
> >
> > > I can imagine to retrieve the MemoryRegion from guest phys address,
> > > then check the owner is TYPE_NVDIMM for example. Is this a good
> > > solution?
> >
> > No, I think it's upto whatever creates the region to set a flag
> > somewhere properly - there's no telling whether it'll always be NVDIMM
> > or some other object.
> 
> We could make the owner object set a flag on the MemoryRegion, or
> implement a common NV interface.

I think preferably on the RAMBlock, but yes; the question then is
whether we need to split the PMEM flag into two for (a) and (b) above
and whether they need to be separately set.

Dave

> >
> > Dave
> >
> > > There is memory_region_from_host(), is there a memory_region_from_guest() 
> > > ?
> > >
> > > thanks
> > >
> > >
> > > --
> > > Marc-André Lureau
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
> 
> 
> -- 
> Marc-André Lureau
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK



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