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

[Marc-André Lureau]

Hi

On Wed, Sep 19, 2018 at 4:14 PM Dr. David Alan Gilbert
<address@hidden> wrote:
>
> * Igor Mammedov (address@hidden) wrote:
> > On Wed, 19 Sep 2018 11:58:22 +0100
> > "Dr. David Alan Gilbert" <address@hidden> wrote:
> >
> > > * 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.
> > well,
> > I get current pmem flag semantics as backend supports persistent memory
> > whether frontend will use it or not is different story.
> >
> > Perhaps we need a separate flag which say that pmem is in use,
> > but what about usecase where nvdimm is used as RAM and not storage
> > we probably would like to wipe it out as normal RAM.
>
> Right, so we're slowly building up the number of flags/policies we're
> trying to represent here; it's certainly not the one 'pmem' flag we
> currently have.
>
> > Maybe we should add frontend property to allow selecting policy per device,
> > which then could be propagated to MemoryRegion/RAMBlock?
>
> By 'device' here you mean memory-backend-* objects? If so then yes I'd
> agree a property on those propagated to the underlying RAMBlock would
> be ideal.

Earlier I was trying to suggest the guest exposed device / owner
(dimm/nvdimm etc) would be the one setting a flag. I can work on a
patch, that should be more clear.

> Dave
>
> > > 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
> >
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK



-- 
Marc-André Lureau