Re: [Xen-devel] [PATCH] libxc: try to find last used pfn when migrating

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Nov 27, 2015 at 03:50:53PM +0100, Juergen Gross wrote:
> For migration the last used pfn of a guest is needed to size the
> logdirty bitmap and as an upper bound of the page loop. Unfortunately
> there are pv-kernels advertising a much higher maximum pfn as they
> are really using in order to support memory hotplug. This will lead
> to allocation of much more memory in Xen tools during migration as
> really needed.
> 
> Try to find the last used guest pfn of a pv-domu by scanning the p2m
> tree from the last entry towards it's start and search for an entry
> not being invalid.
> 
> Normally the mid pages of the p2m tree containing all invalid entries
> are being reused, so we can just scan the top page for identical
> entries and skip them but the first one.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  tools/libxc/xc_sr_save.c        |  8 ++++----
>  tools/libxc/xc_sr_save_x86_pv.c | 34 +++++++++++++++++++++++++++++++---
>  2 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/libxc/xc_sr_save.c b/tools/libxc/xc_sr_save.c
> index 0c12e56..22b3f18 100644
> --- a/tools/libxc/xc_sr_save.c
> +++ b/tools/libxc/xc_sr_save.c
> @@ -677,6 +677,10 @@ static int setup(struct xc_sr_context *ctx)
>      DECLARE_HYPERCALL_BUFFER_SHADOW(unsigned long, dirty_bitmap,
>                                      &ctx->save.dirty_bitmap_hbuf);
>  
> +    rc = ctx->save.ops.setup(ctx);
> +    if ( rc )
> +        goto err;
> +
>      dirty_bitmap = xc_hypercall_buffer_alloc_pages(
>                     xch, dirty_bitmap, 
> NRPAGES(bitmap_size(ctx->save.p2m_size)));
>      ctx->save.batch_pfns = malloc(MAX_BATCH_SIZE *
> @@ -692,10 +696,6 @@ static int setup(struct xc_sr_context *ctx)
>          goto err;
>      }
>  

p2m_size is set in two places for PV guest.  Since we are now relying on
ops.setup to set p2m_size, the invocation to xc_domain_nr_gpfns in
xc_domain_save should be removed, so that we only have one place to set
p2m_size.

You then also need to call xc_domain_nr_gpfns in HVM's ops.setup
(x86_hvm_setup).

Wei.

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