Re: [PATCH] virtio-mem: Fix the bitmap index of the section offset

[David Hildenbrand]

On 16.12.22 07:22, Chenyi Qiang wrote:
> vmem->bitmap indexes the memory region of the virtio-mem backend at a
> granularity of block_size. To calculate the index of target section offset,
> the block_size should be divided instead of the bitmap_size.

I'm curious, what's the user-visible effect and how did you identify 
this issue?

IIUC, we could end up our search for a plugged/unplugged block "too 
late", such that we miss to process blocks.

That would be the case if the bitmap_size < block_size, which should 
effectively always happen ...


unplug_all and migration would be affected, which is why a simple test 
case without a guest reboot/migration wouldn't run into it.

> Fixes: 2044969f0b ("virtio-mem: Implement RamDiscardManager interface")
> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
> ---
>   hw/virtio/virtio-mem.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index ed170def48..e19ee817fe 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -235,7 +235,7 @@ static int virtio_mem_for_each_plugged_section(const 
> VirtIOMEM *vmem,
>       uint64_t offset, size;
>       int ret = 0;
>   
> -    first_bit = s->offset_within_region / vmem->bitmap_size;
> +    first_bit = s->offset_within_region / vmem->block_size;
>       first_bit = find_next_bit(vmem->bitmap, vmem->bitmap_size, first_bit);
>       while (first_bit < vmem->bitmap_size) {
>           MemoryRegionSection tmp = *s;
> @@ -267,7 +267,7 @@ static int virtio_mem_for_each_unplugged_section(const 
> VirtIOMEM *vmem,
>       uint64_t offset, size;
>       int ret = 0;
>   
> -    first_bit = s->offset_within_region / vmem->bitmap_size;
> +    first_bit = s->offset_within_region / vmem->block_size;
>       first_bit = find_next_zero_bit(vmem->bitmap, vmem->bitmap_size, 
> first_bit);
>       while (first_bit < vmem->bitmap_size) {
>           MemoryRegionSection tmp = *s;

Looks correct to me

Reviewed-by: David Hildenbrand <david@redhat.com>

Thanks!

--
Thanks,

David / dhildenb