Re: [PULL 06/30] softmmu/dirtylimit: Implement virtual CPU throttle

[Peter Maydell]

On Wed, 20 Jul 2022 at 12:30, Dr. David Alan Gilbert (git)
<dgilbert@redhat.com> wrote:
>
> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>
> Setup a negative feedback system when vCPU thread
> handling KVM_EXIT_DIRTY_RING_FULL exit by introducing
> throttle_us_per_full field in struct CPUState. Sleep
> throttle_us_per_full microseconds to throttle vCPU
> if dirtylimit is in service.
>
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Message-Id: 
> <977e808e03a1cef5151cae75984658b6821be618.1656177590.git.huangy81@chinatelecom.cn>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>


Hi; Coverity points out a problem with this code (CID 1490787):

> +static inline int64_t dirtylimit_dirty_ring_full_time(uint64_t dirtyrate)
> +{
> +    static uint64_t max_dirtyrate;
> +    uint32_t dirty_ring_size = kvm_dirty_ring_size();
> +    uint64_t dirty_ring_size_meory_MB =
> +        dirty_ring_size * TARGET_PAGE_SIZE >> 20;

Because dirty_ring_size and TARGET_PAGE_SIZE are both 32 bits,
this multiplication will be done as a 32-bit operation,
which could overflow. You should cast one of the operands
to uint64_t to ensure that the operation is done as a 64 bit
multiplication.

Side note: typo in the variable name: should be 'memory'.


> +    if (max_dirtyrate < dirtyrate) {
> +        max_dirtyrate = dirtyrate;
> +    }
> +
> +    return dirty_ring_size_meory_MB * 1000000 / max_dirtyrate;
> +}

thanks
-- PMM