Re: [Qemu-devel] [PATCH] ppc64: fix compressed dump with pseries kernel

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Aug 05, 2016 at 11:26:47AM +0200, Andrew Jones wrote:
> On Thu, Aug 04, 2016 at 10:41:16AM +0200, Laurent Vivier wrote:
> > 
> > 
> > On 04/08/2016 04:38, David Gibson wrote:
> > > On Wed, Aug 03, 2016 at 09:55:07PM +0200, Laurent Vivier wrote:
> > >> If we don't provide the page size in target-ppc:cpu_get_dump_info(),
> > >> the default one (TARGET_PAGE_SIZE, 4KB) is used to create
> > >> the compressed dump. It works fine with Macintosh, but not with
> > >> pseries as the kernel default page size is 64KB.
> > >>
> > >> Without this patch, if we generate a compressed dump in the QEMU monitor:
> > >>
> > >>     (qemu) dump-guest-memory -z qemu.dump
> > >>
> > >> This dump cannot be read by crash:
> > >>
> > >>     # crash vmlinux qemu.dump
> > >>     ...
> > >>     WARNING: cannot translate vmemmap kernel virtual addresses:
> > >>              commands requiring page structure contents will fail
> > >>     ...
> > >>
> > >> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > >> ---
> > >>  target-ppc/arch_dump.c | 5 +++++
> > >>  1 file changed, 5 insertions(+)
> > > 
> > > Urgh.. so, really the page size used by the guest kernel is a
> > > guest-side detail, and it's certainly possible to build a 4kiB page
> > > guest kernel, although 64kiB is the norm.
> > 
> > virtio-balloon doesn't work with 4K kernel.
> > 
> > > This might be the best we can do, but it'd be nice if we could probe
> > > or otherwise avoid relying on this assumption about the guest kernel.
> > 
> > I agree with you but none of the other architectures probes for the page
> > size.
> > 
> > For instance ARM: |I cc: Drew to know how he has chosen the values]
> > 
> >     if (arm_feature(env, ARM_FEATURE_AARCH64)) {
> > ...
> >         info->page_size = (1 << 16);
> > ...
> >     } else {
> > ...
> >         info->page_size = (1 << 12);
> > ...
> >     }
> >
> 
> info->page_size is used to determine the dumpfile's block size. The
> block size needs to be at least the page size, but a multiple of page
> size works fine too. As we can't probe for the currently used guest
> page size, and a multiple of page size is fine, then using the guest's
> maximum allowed page size is the best we can do.

Aha!  Well that makes life much easier.  I'll adjust the commit
message and apply.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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