Re: [Qemu-devel] [PATCH] migration: avoid copying ignore-shared ramblock when in incoming migration

[Peter Maydell]

On Thu, 21 Mar 2019 at 15:32, Catherine Ho <address@hidden> wrote:
> So in igore-shared case, dtb is not required here ?

Can you explain what the "ignore-shared" case is?

In general, when we reset the system, we want to bring it
back to exactly the state that it was in when QEMU was
first started. That means we need to reload all the rom blob
data into memory (because the guest might have modified
that memory while it was running).

If I understand correctly from other threads, the idea is
that some of the RAM is shared between source and destination
so it does not need to be manually copied during migration.
If that is correct, then perhaps the right thing is that
in the rom_reset code:
 * if this rom blob is being loaded into a "shared" ram block
 * and this reset is happening specifically before an
   inbound migration
 * then skip loading the rom blob data

?

I don't know the right way to determine either the "is this
a shared ram area" or "is this the reset prior to inbound
migration", but perhaps you can fill that in.

> Maybe I could add a flag in struct Rom to set it when the rom is created by
> rom_add_blob_fixed_as()? And in rom_reset, just bypass this rom when
> in_incoming_migration && ignore_shared

No, I think this is wrong -- the particular function used
to create the rom blob data should not affect how we are
treating it.

thanks
-- PMM