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

[Juergen Gross <jgross@xxxxxxxx>]

On 27/11/15 16:33, Wei Liu wrote:
> 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.

I wasn't sure this is needed in save case only. If it is I can make
the change, of course. Andrew?

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

Sure.


Juergen


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