Re: [Qemu-devel] [PATCH v3 2/5] util: introduce threaded workqueue

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Xiao Guangrong (guangrong.xiao@gmail.com) wrote:
> 
> 
> On 11/23/18 7:02 PM, Dr. David Alan Gilbert wrote:
> 
> > > +#include "qemu/osdep.h"
> > > +#include "qemu/bitmap.h"
> > > +#include "qemu/threaded-workqueue.h"
> > > +
> > > +#define SMP_CACHE_BYTES 64
> > 
> > That's architecture dependent isn't it?
> > 
> 
> Yes, it's arch dependent indeed.
> 
> I just used 64 for simplification and i think it is <= 64 on most CPU arch-es
> so that can work.
> 
> Should i introduce statically defined CACHE LINE SIZE for all arch-es? :(

I think it depends why you need it; but we shouldn't have a constant
that is wrong, and we shouldn't define something architecture dependent
in here.

> > > +   /*
> > > +     * the bit in these two bitmaps indicates the index of the ＠requests
> > > +     * respectively. If it's the same, the corresponding request is free
> > > +     * and owned by the user, i.e, where the user fills a request. Otherwise,
> > > +     * it is valid and owned by the thread, i.e, where the thread fetches
> > > +     * the request and write the result.
> > > +     */
> > > +
> > > +    /* after the user fills the request, the bit is flipped. */
> > > +    uint64_t request_fill_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES);
> > > +    /* after handles the request, the thread flips the bit. */
> > > +    uint64_t request_done_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES);
> > 
> > Patchew complained about some type mismatches; I think those are because
> > you're using the bitmap_* functions on these; those functions always
> > operate on 'long' not on uint64_t - and on some platforms they're
> > unfortunately not the same.
> 
> I guess you were taking about this error:
> ERROR: externs should be avoided in .c files
> #233: FILE: util/threaded-workqueue.c:65:
> +    uint64_t request_fill_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES);
> 
> The complained thing is "QEMU_ALIGNED(SMP_CACHE_BYTES)" as it gone
> when the aligned thing is removed...
> 
> The issue you pointed out can be avoid by using type-casting, like:
> bitmap_xor(..., (void *)&thread->request_fill_bitmap)
> cannot we?

I thought the error was just due to long vs uint64_t ratehr than the
qemu_aligned.  I don't think it's just a casting problem, since I don't
think the long's are always 64bit.

Dave

> Thanks!
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK