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

[Andrew Jones <drjones@redhat.com>]

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.

Thanks,
drew