Re: [Xen-devel] [RFC v3 5/6] xen/arm: Add log_dirty support for ARM

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Wei,

On 05/08/2014 10:18 PM, Wei Huang wrote:
> This patch implements log_dirty for ARM guest VMs. This feature
> is provided via two basic blocks: dirty_bit_map and VLPT
> (virtual-linear page table)
> 
> 1. VLPT provides fast accessing of 3rd PTE of guest P2M.
> When creating a mapping for VLPT, the page table mapping
> becomes the following:
>    xen's 1st PTE --> xen's 2nd PTE --> guest p2m's 2nd PTE -->
>    guest p2m's 3rd PTE
> 
> With VLPT, xen can immediately locate the 3rd PTE of guest P2M
> and modify PTE attirbute during dirty page tracking. The following
> link shows the performance comparison for handling a dirty-page
> between VLPT and typical page table walking.
> http://lists.xen.org/archives/html/xen-devel/2013-08/msg01503.html
> 
> For more info about VLPT, please see
> http://www.technovelty.org/linux/virtual-linear-page-table.html.
> 
> 2. Dirty bitmap
> The dirty bitmap is used to mark the pages which are dirty during
> migration. The info is used by Xen tools, via DOMCTL_SHADOW_OP_*,
> to figure out which guest pages need to be resent.

I think you forgot a case in the log dirty support. You don't handle
dirty page when Xen is writing data in the guest memory (with
raw_copy_to_guest).

I think we need to handle this case during live migration otherwise data
may be corrupted.

Regards,

-- 
Julien Grall

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