Re: [Qemu-devel] [RFC PATCH v1 1/2] target-arm: Update page size for aarch64

[Vijay Kilari]

On Mon, Apr 4, 2016 at 7:14 PM, Peter Maydell <address@hidden> wrote:
> On 4 April 2016 at 14:39,  <address@hidden> wrote:
>> From: Vijay <address@hidden>
>>
>> Set target page size to minimum 4K for aarch64.
>> This helps to reduce live migration downtime significantly.
>>
>> Signed-off-by: Vijaya Kumar K <address@hidden>
>> ---
>>  target-arm/cpu.h |    7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/target-arm/cpu.h b/target-arm/cpu.h
>> index 066ff67..2e4b48f 100644
>> --- a/target-arm/cpu.h
>> +++ b/target-arm/cpu.h
>> @@ -1562,11 +1562,18 @@ bool write_cpustate_to_list(ARMCPU *cpu);
>>  #if defined(CONFIG_USER_ONLY)
>>  #define TARGET_PAGE_BITS 12
>>  #else
>> +/*
>> + * Aarch64 support minimum 4K page size
>> + */
>> +#if defined(TARGET_AARCH64)
>> +#define TARGET_PAGE_BITS 12
>
> I agree that this would definitely improve performance (both for
> migration and for emulated guests), but I'm afraid this breaks
> running 32-bit ARMv5 and ARMv7M guests with this QEMU binary,
> so we can't do this. If we want to allow the minimum page size to
> be bigger than 1K for AArch64 CPUs then we need to make it a
> runtime settable thing rather than compile-time (which is not
> an entirely trivial thing).

Do you mean to say that based on -cpu type qemu option
choose the page size at runtime?

>
>> +#else
>>  /* The ARM MMU allows 1k pages.  */
>>  /* ??? Linux doesn't actually use these, and they're deprecated in recent
>>     architecture revisions.  Maybe a configure option to disable them.  */
>>  #define TARGET_PAGE_BITS 10
>>  #endif
>> +#endif
>>
>>  #if defined(TARGET_AARCH64)
>>  #  define TARGET_PHYS_ADDR_SPACE_BITS 48
>
> thanks
> -- PMM