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Re: [PATCH 1/2] accel: kvm: Fix memory waste under mismatch page size
From: |
Keqian Zhu |
Subject: |
Re: [PATCH 1/2] accel: kvm: Fix memory waste under mismatch page size |
Date: |
Wed, 16 Dec 2020 16:08:37 +0800 |
User-agent: |
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1 |
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
>
> .
>