Re: [PATCH v10 7/7] multifd: Implement zero copy write in multifd migration (multifd-zero-copy)

[Peter Xu]

On Tue, Apr 26, 2022 at 08:06:56PM -0300, Leonardo Bras wrote:
> 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>

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

-- 
Peter Xu