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 <lviv...@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
