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

[Laurent Vivier]

On 05/08/2016 11:26, 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 <address@hidden>
>>>> ---
>>>>  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.

Thank you for the explanation.

So we can unconditionally use 64KB, even for mac99 with a 64bit
processor or a 32bit processor (that are always 4K page size)?

The maximum page size in the kernel can be 256kB [1], should we use this
value instead?

Laurent

[1] linux/arch/powerpc/include/asm/page.h
/*
 * On regular PPC32 page size is 4K (but we support 4K/16K/64K/256K pages
 * on PPC44x). For PPC64 we support either 4K or 64K software
 * page size. When using 64K pages however, whether we are really supporting
 * 64K pages in HW or not is irrelevant to those definitions.
 */
#if defined(CONFIG_PPC_256K_PAGES)
#define PAGE_SHIFT              18
#elif defined(CONFIG_PPC_64K_PAGES)
#define PAGE_SHIFT              16
#elif defined(CONFIG_PPC_16K_PAGES)
#define PAGE_SHIFT              14
#else
#define PAGE_SHIFT              12
#endif