Re: [PATCH v2 2/2] accel: kvm: Add aligment assert for kvm_log_clear_one_slot

[Thomas Huth]

On 17/12/2020 02.49, Keqian Zhu wrote:
> The parameters start and size are transfered from QEMU memory
> emulation layer. It can promise that they are TARGET_PAGE_SIZE
> aligned. However, KVM needs they are qemu_real_page_size aligned.
>
> Though no caller breaks this aligned requirement currently, we'd
> better add an explicit assert to avoid future breaking.
>
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> ---
>   accel/kvm/kvm-all.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> ---
> v2
>   - Address Andrew's commment (Use assert instead of return err).
>
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index f6b16a8df8..73b195cc41 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -692,6 +692,10 @@ out:
>   #define KVM_CLEAR_LOG_ALIGN  (qemu_real_host_page_size << KVM_CLEAR_LOG_SHIFT)
>   #define KVM_CLEAR_LOG_MASK   (-KVM_CLEAR_LOG_ALIGN)
>   
> +/*
> + * As the granule of kvm dirty log is qemu_real_host_page_size,
> + * @start and @size are expected and restricted to align to it.
> + */
>   static int kvm_log_clear_one_slot(KVMSlot *mem, int as_id, uint64_t start,
>                                     uint64_t size)
>   {
> @@ -701,6 +705,9 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int as_id, 
> uint64_t start,
>       unsigned long *bmap_clear = NULL, psize = qemu_real_host_page_size;
>       int ret;
>   
> +    /* Make sure start and size are qemu_real_host_page_size aligned */
> +    assert(QEMU_IS_ALIGNED(start | size, psize));

Sorry, but that was a bad idea: It triggers and kills my Centos 6 VM:

$ qemu-system-x86_64 -accel kvm -hda ~/virt/images/centos6.qcow2 -m 1G
qemu-system-x86_64: ../../devel/qemu/accel/kvm/kvm-all.c:690: 
kvm_log_clear_one_slot: Assertion `QEMU_IS_ALIGNED(start | size, psize)' failed.
Aborted (core dumped)

Can we please revert this patch?

 Thomas