Re: [PATCH 12/16] hw/block/nvme: refactor NvmeRequest clearing

[Klaus Jensen]

On Jul 29 20:47, Maxim Levitsky wrote:
> On Mon, 2020-07-20 at 13:37 +0200, Klaus Jensen wrote:
> > From: Klaus Jensen <k.jensen@samsung.com>
> > 
> > Move clearing of the structure from "clear before use" to "clear
> > after use".
> > 
> > Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
> > ---
> >  hw/block/nvme.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/block/nvme.c b/hw/block/nvme.c
> > index e2932239c661..431f26c2f589 100644
> > --- a/hw/block/nvme.c
> > +++ b/hw/block/nvme.c
> > @@ -209,6 +209,11 @@ static void nvme_irq_deassert(NvmeCtrl *n, NvmeCQueue 
> > *cq)
> >      }
> >  }
> >  
> > +static void nvme_req_clear(NvmeRequest *req)
> > +{
> > +    memset(&req->cqe, 0x0, sizeof(req->cqe));
> > +}
> > +
> >  static uint16_t nvme_map_addr_cmb(NvmeCtrl *n, QEMUIOVector *iov, hwaddr 
> > addr,
> >                                    size_t len)
> >  {
> > @@ -458,6 +463,7 @@ static void nvme_post_cqes(void *opaque)
> >          nvme_inc_cq_tail(cq);
> >          pci_dma_write(&n->parent_obj, addr, (void *)&req->cqe,
> >              sizeof(req->cqe));
> > +        nvme_req_clear(req);
> 
> Don't we need some barrier here to avoid reordering the writes?
> pci_dma_write does seem to include a barrier prior to the write it does
> but not afterward.
> 


> Also what is the motivation of switching the order?

This was just preference. But I did not consider that I would be
breaking any DMA rules here.

> I think somewhat that it is a good thing to clear a buffer,
> before it is setup.
> 

I'll reverse my preference and keep the status quo since I have no
better motivation than personal preference.

The introduction of nvme_req_clear is just in preparation for
consolidating more cleanup here, but I'll drop this patch and introduce
nvme_req_clear later.