Re: [PATCH 1/1] iommu/iova: Make rcache flush optional on IOVA allocation failure

[Tomasz Nowicki]

Hi Robin,

On 18.09.2017 18:02, Robin Murphy wrote:
> Hi Tomasz,
>
> On 18/09/17 11:56, Tomasz Nowicki wrote:
> > Since IOVA allocation failure is not unusual case we need to flush
> > CPUs' rcache in hope we will succeed in next round.
> >
> > However, it is useful to decide whether we need rcache flush step because
> > of two reasons:
> > - Not scalability. On large system with ~100 CPUs iterating and flushing
> >    rcache for each CPU becomes serious bottleneck so we may want to deffer it.
> > - free_cpu_cached_iovas() does not care about max PFN we are interested in.
> >    Thus we may flush our rcaches and still get no new IOVA like in the
> >    commonly used scenario:
> >
> >      if (dma_limit > DMA_BIT_MASK(32) && dev_is_pci(dev))
> >          iova = alloc_iova_fast(iovad, iova_len, DMA_BIT_MASK(32) >> shift);
> >
> >      if (!iova)
> >          iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift);
> >
> >     1. First alloc_iova_fast() call is limited to DMA_BIT_MASK(32) to get
> >        PCI devices a SAC address
> >     2. alloc_iova() fails due to full 32-bit space
> >     3. rcaches contain PFNs out of 32-bit space so free_cpu_cached_iovas()
> >        throws entries away for nothing and alloc_iova() fails again
> >     4. Next alloc_iova_fast() call cannot take advantage of rcache since we
> >        have just defeated caches. In this case we pick the slowest option
> >        to proceed.
> >
> > This patch reworks flushed_rcache local flag to be additional function
> > argument instead and control rcache flush step. Also, it updates all users
> > to do the flush as the last chance.
>
> Looks like you've run into the same thing Nate found[1] - I came up with
> almost the exact same patch, only with separate alloc_iova_fast() and
> alloc_iova_fast_noretry() wrapper functions, but on reflection, just
> exposing the bool to callers is probably simpler. One nit, can you
> document it in the kerneldoc comment too? With that:
>
> Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

Thanks! I will add missing comment.

Tomasz