Re: [Qemu-devel] [PATCH v4 07/22] vhost: alloc shareable log

[Marc-André Lureau]

Hi

On Tue, Sep 22, 2015 at 12:12 PM, Michael S. Tsirkin <address@hidden> wrote:
>> But then it should also VHOST_SET_LOG_BASE all the other devices with
>> the new log, unless you want to tackle only the future log users. So
>> it needs to track all the users of the log.
>
> We already do this.
> Same applies to non-memfd->memfd switch.
>

I failed to see how the log of other devices is being updated when one
device needs a resize. There are multiple log already in this case.

>> Is there a clear benefit
>> of this? since the memory isn't shared without the memfd passed to
>> another process and the overhead of memfd is probably quite small, and
>> pre-shm or future resize will not use the shared memory already.
>
> For example, THP doesn't work for memfd at the moment,
> so all accesses are a bit slower.

What's THP? How is it slower once the fd is mmap?

> Really, I don't want to merge hacks. Switching from non memfd
> to memfd but not back has all the signs of one.
> Let's do it cleanly please.

The current code isn't switching existing logs either. What would be
simpler to do is to allocate a different log region for the "share"
case. So there would be no need to switch other non-shareable logs (to
the cost of using twice the memory needed).

-- 
Marc-André Lureau