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

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

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On Thu, Aug 04, 2016 at 10:41:16AM +0200, Laurent Vivier wrote:
1;4402;0c> 
> 
> 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.

It doesn't?  Balloon has rather a lot of flaws, but I didn't think
that was one of them.

> > 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.

Yeah :/

> 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);
> ...
>     }
> 
> In the kernel:
> 
> arch/arm64/include/asm/page.h:
> 
> #define PAGE_SHIFT		CONFIG_ARM64_PAGE_SHIFT
> 
> arch/arm64/Kconfig:
> 
> config ARM64_PAGE_SHIFT
>         int
>         default 16 if ARM64_64K_PAGES
>         default 14 if ARM64_16K_PAGES
>         default 12
> 
> choice
>         prompt "Page size"
>         default ARM64_4K_PAGES
>         help
>           Page size (translation granule) configuration.
> 
> config ARM64_4K_PAGES
>         bool "4KB"
>         help
>           This feature enables 4KB pages support.
> 
> config ARM64_16K_PAGES
>         bool "16KB"
>         help
>           The system will use 16KB pages support. AArch32 emulation
>           requires applications compiled with 16K (or a multiple of 16K)
>           aligned segments.
> 
> config ARM64_64K_PAGES
>         bool "64KB"
>         help
>           This feature enables 64KB pages support (4KB by default)
>           allowing only two levels of page tables and faster TLB
>           look-up. AArch32 emulation requires applications compiled
>           with 64K aligned segments.
> 
> endchoice
> 
> I think we can't rely on the CPU state or the memory content as they can
> be corrupted.

I guess.  I don't know that we can really get what we want from there
anyway, at least not without even more assumptions about the guest
state than.

Hrm.  I guess I'm ok with the change, but I'd like the commit message
updated to recognize that this is a compromise just designed to work
with the most common guests.

-- 
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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