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

[Laurent Vivier <lvivier@redhat.com>]

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);
...
    }

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.

Thanks,
Laurent