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Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping fre

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages
Date: Thu, 24 Mar 2016 16:47:36 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 24/03/2016 16:39, Li, Liang Z wrote:
> > Only if you write the arch specific thing for all arches.
> I plan to keep a function stub for each arch to implement. And I
> have done that for X86.

Again: the ram_addr_t matching is internal to QEMU and can vary from
release to release.  Do not do this.

> > I think the reason for the speedup that you observe is that you only need to
> > translate ram_addr_t to GPA once per ramblock, which is much faster than
> > translating GPA to ram_addr_t for each page.
> Yes, exactly! 

You don't need to translate it once per page.  When QEMU copies the
bitmap from guest memory to its own internal data structures, it can do
so one block at a time with a function like

    void bitmap_copy_bits(unsigned long *dst, unsigned int dst_start,
                          unsigned long *src, unsigned int src_start
                          unsigned int nbits);


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