Re: [PATCH] MIPS: CPU_LOONGSON2EF need software to maintain cache consistency

[Thomas Bogendoerfer <tsbogend@alpha.franken.de>]

On Tue, May 26, 2020 at 11:25:56PM +0800, Jiaxun Yang wrote:
> On Tue, 26 May 2020 14:29:50 +0100
> Robin Murphy <robin.murphy@arm.com> wrote:
> 
> > On 2020-05-26 14:01, Thomas Bogendoerfer wrote:
> > > On Tue, May 26, 2020 at 08:40:28PM +0800, Lichao Liu wrote:  
> > >> Loongson-2EF need software maintain cache consistency, So when
> > >> using streaming DMA, software needs to maintain consistency.
> > >>
> > >> dma_map_single() is correct, but dma_unmap_single is wrong.
> > >>
> > >> The function call path:
> > >> 'dma_unmap_single->dma_unmap_page_attrs->dma_direct_unmap_page->
> > >>   dma_direct_sync_single_for_cpu->arch_sync_dma_for_cpu->
> > >>   cpu_needs_post_dma_flush'
> > >>
> > >> In current version, 'cpu_needs_post_dma_flush' will return false
> > >> at Loongon-2EF platform, and dma_unmap_single will not invalidate
> > >> cache, driver may access wrong dma data.  
> > > 
> > > why should it ? CPU must not touch data while it's mapped for DMA.
> > >   
> > >> I don't know what's the exact meaning of "fill random cachelines
> > >> with stale data at any time". I always think
> > >> 'cpu_needs_post_dma_flush()' means whether this platform needs
> > >> software to maintain cache consistency.  
> > > 
> > > this will only happen, if cpu speculates creates dirty cache lines
> > > by speculation as R10k type of CPUs do.  
> > 
> > Will it? The usual pattern for this problem is that the CPU 
> > speculatively fills a (clean) cache line after a DMA_FROM_DEVICE 
> > operation has begun, but before the device has actually written to
> > that part of the buffer. Thus a subsequent CPU read after the
> > operation is complete can hit in the cache and return the previous
> > data rather than the updated data that the device wrote. I don't know
> > about MIPS specifically, but that can certainly happen on Arm.
> 
> Checked the manual of Loongson-2F again and I must admit the case may
> happen on Loongson-2EF processor.

so the patch is correct ?
> 
> R4k manual didn't show the details of speculative policy but I think
> that should be applied to all R4k like processors?

R4k doesn't speculate at all.

Thomas.

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