Re: [PATCH 1/2] accel: kvm: Fix memory waste under mismatch page size

[Keqian Zhu]

Hi drew,

On 2020/12/15 19:20, Andrew Jones wrote:
> On Tue, Dec 15, 2020 at 03:19:47PM +0800, Keqian Zhu wrote:
>> When handle dirty log, we face qemu_real_host_page_size and
>> TARGET_PAGE_SIZE. The first one is the granule of KVM dirty
>> bitmap, and the second one is the granule of QEMU dirty bitmap.
>>
>> Generally speaking, qemu_real_host_page_size >= TARGET_PAGE_SIZE,
> 
> Not just generally speaking, but must be. We have
> 
>     /*
>      * On systems where the kernel can support different base page
>      * sizes, host page size may be different from TARGET_PAGE_SIZE,
>      * even with KVM.  TARGET_PAGE_SIZE is assumed to be the minimum
>      * page size for the system though.
>      */
>     assert(TARGET_PAGE_SIZE <= qemu_real_host_page_size);
Yes, I noticed it, but my statement (Generally speaking) is not suitable.
Thanks for pointing it out.

> 
> at the top of kvm_init() to enforce it.
> 
> The comment also says TARGET_PAGE_SIZE is assumed to be the minimum,
> which is more of a requirement than assumption. And, that requirement
> implies that we require all memory regions and base addresses to be
> aligned to the maximum possible target page size (in case the target
> actually uses something larger).
> 
> Please remove 'Generally speaking' from the commit message. It
> implies this change is based on an assumption rather than a rule.
> 
> (It'd be nice if we had these guest memory and TARGET_PAGE_SIZE
> requirements better documented and in one place.)
Maybe someone could do this :-)

> 
>> so misuse TARGET_PAGE_SIZE to init kvmslot dirty_bmap may waste
>> memory. For example, when qemu_real_host_page_size is 64K and
>> TARGET_PAGE_SIZE is 4K, this bugfix can save 93.75% memory.
>>
>> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
>> ---
>>  accel/kvm/kvm-all.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>> index baaa54249d..c5e06288eb 100644
>> --- a/accel/kvm/kvm-all.c
>> +++ b/accel/kvm/kvm-all.c
>> @@ -620,8 +620,12 @@ static void kvm_memslot_init_dirty_bitmap(KVMSlot *mem)
>>       * too, in most cases).
>>       * So for now, let's align to 64 instead of HOST_LONG_BITS here, in
>>       * a hope that sizeof(long) won't become >8 any time soon.
>> +     *
>> +     * Note: the granule of kvm dirty log is qemu_real_host_page_size.
>> +     * And mem->memory_size is aligned to it (otherwise this mem can't
>> +     * be registered to KVM).
>>       */
>> -    hwaddr bitmap_size = ALIGN(((mem->memory_size) >> TARGET_PAGE_BITS),
>> +    hwaddr bitmap_size = ALIGN(mem->memory_size / qemu_real_host_page_size,
>>                                          /*HOST_LONG_BITS*/ 64) / 8;
>>      mem->dirty_bmap = g_malloc0(bitmap_size);
>>  }
>> -- 
>> 2.23.0
>>
>>
> 
> Besides the commit message
> 
> Reviewed-by: Andrew Jones <drjones@redhat.com>
> 
OK, I will correct it and send v2 soon.

Cheers,
Keqian
> 
> Thanks,
> drew
> 
> .
>