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multifd: Implement zero copy write in multifd migration (multifd-zero…
…-copy) Implement zero copy send on nocomp_send_write(), by making use of QIOChannel writev + flags & flush interface. Change multifd_send_sync_main() so flush_zero_copy() can be called after each iteration in order to make sure all dirty pages are sent before a new iteration is started. It will also flush at the beginning and at the end of migration. Also make it return -1 if flush_zero_copy() fails, in order to cancel the migration process, and avoid resuming the guest in the target host without receiving all current RAM. This will work fine on RAM migration because the RAM pages are not usually freed, and there is no problem on changing the pages content between writev_zero_copy() and the actual sending of the buffer, because this change will dirty the page and cause it to be re-sent on a next iteration anyway. A lot of locked memory may be needed in order to use multifd migration with zero-copy enabled, so disabling the feature should be necessary for low-privileged users trying to perform multifd migrations. Signed-off-by: Leonardo Bras <leobras@redhat.com> Message-Id: <20220426230654.637939-8-leobras@redhat.com> Reviewed-by: Peter Xu <peterx@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
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