Re: [Xen-devel] [PATCH for-4.6] libxl: move calling libxl__arch_domain_construct_memmap to right place

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Aug 05, 2015 at 12:06:22PM +0100, Ian Campbell wrote:
> On Wed, 2015-08-05 at 11:58 +0100, Wei Liu wrote:
> > On Wed, Aug 05, 2015 at 11:48:55AM +0100, Ian Campbell wrote:
> > > On Wed, 2015-08-05 at 11:43 +0100, Wei Liu wrote:
> > > > On Wed, Aug 05, 2015 at 11:38:54AM +0100, Ian Campbell wrote:
> > > > > On Wed, 2015-08-05 at 11:23 +0100, Wei Liu wrote:
> > > > > > This function was called in the wrong place, because both
> > > > > > libxl__vnuma_build_vmemrange_hvm and xc_hvm_build rely on its 
> > > > > > output.
> > > > > 
> > > > > What is the effect of this call being in the wrong place? 
> > > > > Presumably 
> > > > > one or
> > > > > the other of those functions reaches the wrong conclusion?
> > > > 
> > > > Originally, by the time that function got called, all guest pages 
> > > > were
> > > > already populated.  The end result is E820 map disagrees with what 
> > > > vNUMA
> > > > says and what address ranges memory actually resides, i.e. risk of 
> > > > guest
> > > > accessing region that doesn't have backing pages.
> > > 
> > > Ouch. This should certainly be explained in the commit message.
> > > 
> > > With that: Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > > 
> > 
> > I will post v2 shortly with that information in commit message.
> > 
> > > Although perhaps we should wait for confirmation this fix doesn't 
> > > regress
> > > RMRR somehow?
> > > 
> > 
> > I doubt it will. The code as-is is already broken for RMRR. But let's
> > wait till tomorrow for Tiejun to reply.
> > 
> > If he doesn't reply by tomorrow, I suggest we apply v2 first and
> > fix up any subsequent issues later.
> 
> Agreed.

Actually I want to retract this patch. I confused hvm path with pv path
and drew my conclusion when looking at both code paths.

In hvm path, neither libxl__vnuma_build_vmemragen_hvm nor xc_hvm_build
depends on output of libxl__arch_domain_construct_memmap (in fact it
doesn't change anything). So the code is OK.

In pv path, there is a path which relies on having a valid E820 map
first, but that path 1) relies on host E820 map; 2) doesn't involve RMRR
support.

In the end, moving that function call has no effect whatsoever.

Sorry for the noise!

Wei.

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