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

[Laszlo Ersek <lersek@redhat.com>]

+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
> <dgilbert@redhat.com> 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.)

Laszlo