Re: [PATCH resend v2 5/5] softmmu/memory_mapping: optimize for RamDiscardManager sections

[Peter Xu <peterx@redhat.com>]

On Tue, Jul 20, 2021 at 03:03:04PM +0200, David Hildenbrand wrote:
> virtio-mem logically plugs/unplugs memory within a sparse memory region
> and notifies via the RamDiscardManager interface when parts become
> plugged (populated) or unplugged (discarded).
> 
> Currently, we end up (via the two users)
> 1) zeroing all logically unplugged/discarded memory during TPM resets.
> 2) reading all logically unplugged/discarded memory when dumping, to
>    figure out the content is zero.
> 
> 1) is always bad, because we assume unplugged memory stays discarded
>    (and is already implicitly zero).
> 2) isn't that bad with anonymous memory, we end up reading the zero
>    page (slow and unnecessary, though). However, once we use some
>    file-backed memory (future use case), even reading will populate memory.
> 
> Let's cut out all parts marked as not-populated (discarded) via the
> RamDiscardManager. As virtio-mem is the single user, this now means that
> logically unplugged memory ranges will no longer be included in the
> dump, which results in smaller dump files and faster dumping.
> 
> virtio-mem has a minimum granularity of 1 MiB (and the default is usually
> 2 MiB). Theoretically, we can see quite some fragmentation, in practice
> we won't have it completely fragmented in 1 MiB pieces. Still, we might
> end up with many physical ranges.
> 
> Both, the ELF format and kdump seem to be ready to support many
> individual ranges (e.g., for ELF it seems to be UINT32_MAX, kdump has a
> linear bitmap).
> 
> Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: Alex Williamson <alex.williamson@redhat.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Claudio Fontana <cfontana@suse.de>
> Cc: Thomas Huth <thuth@redhat.com>
> Cc: "Alex Bennée" <alex.bennee@linaro.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Laurent Vivier <lvivier@redhat.com>
> Cc: Stefan Berger <stefanb@linux.ibm.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu