linux-kernel - Re: [PATCH v2 2/2] xen/arm: introduce GNTTABOP_cache

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Message-ID: <alpine.DEB.2.02.1410031717420.17038@kaball.uk.xensource.com>
Date:	Fri, 3 Oct 2014 17:20:08 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <konrad.wilk@...cle.com>,
	<Ian.Campbell@...rix.com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/2] xen/arm: introduce GNTTABOP_cache_flush

On Fri, 3 Oct 2014, David Vrabel wrote:
> On 03/10/14 15:53, Stefano Stabellini wrote:
> > Introduce support for new hypercall GNTTABOP_cache_flush.
> > Use it to perform cache flashing on pages used for dma when necessary.
> [..]
> >  /* functions called by SWIOTLB */
> > @@ -22,16 +25,31 @@ static void dma_cache_maint(dma_addr_t handle, unsigned long offset,
> >  		size_t len = left;
> >  		void *vaddr;
> >  	
> > +		if (len + offset > PAGE_SIZE)
> > +			len = PAGE_SIZE - offset;
> > +
> >  		if (!pfn_valid(pfn))
> >  		{
> > -			/* TODO: cache flush */
> > +			struct gnttab_cache_flush cflush;
> > +
> > +			cflush.op = 0;
> > +			cflush.a.dev_bus_addr = pfn << PAGE_SHIFT;
> > +			cflush.offset = offset;
> > +			cflush.size = len;
> > +
> > +			if (op == dmac_unmap_area && dir != DMA_TO_DEVICE)
> > +				cflush.op = GNTTAB_CACHE_INVAL;
> > +			if (op == dmac_map_area) {
> > +				cflush.op = GNTTAB_CACHE_CLEAN;
> > +				if (dir == DMA_FROM_DEVICE)
> > +					cflush.op |= GNTTAB_CACHE_INVAL;
> > +			}
> 
> Are all these cache operations needed?  You do a clean on map regardless
> of the direction and INVAL on map seems unnecessary.
> 
> I would have thought it would be:
> 
> map && (TO_DEVICE || BOTH)
>    op = CLEAN
> 
> unmap && (FROM_DEVICE || BOTH)
>    op = INVAL

I was trying to do the same thing Linux is already doing on native to
stay on the safe side.

See arch/arm/mm/cache-v7.S:v7_dma_map_area and
arch/arm/mm/cache-v7.S:v7_dma_unmap_area.

Unless I misread the assembly they should match.


> > --- a/include/xen/interface/grant_table.h
> > +++ b/include/xen/interface/grant_table.h
> > @@ -479,6 +479,24 @@ struct gnttab_get_version {
> >  DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
> >  
> >  /*
> > + * Issue one or more cache maintenance operations on a portion of a
> > + * page granted to the calling domain by a foreign domain.
> > + */
> > +#define GNTTABOP_cache_flush          12
> > +struct gnttab_cache_flush {
> > +    union {
> > +        uint64_t dev_bus_addr;
> > +        grant_ref_t ref;
> > +    } a;
> > +    uint32_t offset; /* offset from start of grant */
> > +    uint32_t size;   /* size within the grant */
> > +#define GNTTAB_CACHE_CLEAN      (1<<0)
> > +#define GNTTAB_CACHE_INVAL      (1<<1)
> > +    uint32_t op;
> > +};
> > +DEFINE_GUEST_HANDLE_STRUCT(gnttab_cache_flush);
> > +
> > +/*
> >   * Bitfield values for update_pin_status.flags.
> >   */
> >   /* Map the grant entry for access by I/O devices. */
> 
> If the ARM bits are correct, have a
> 
> Reviewed-by: David Vrabel <david.vrabel@...rix.com>
> 
> for the common header.
> 
> David
> 
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