From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH kvm-unit-tests 01/17] x86: intel-iommu: add vt-d init test
Date: Tue, 8 Nov 2016 10:24:18 -0500
Message-ID: <20161108152418.GD2793@pxdev.xzpeter.org>
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To: Alexander Gordeev <agordeev@redhat.com>
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On Tue, Nov 08, 2016 at 11:52:35AM +0100, Alexander Gordeev wrote:
> On Wed, Oct 26, 2016 at 03:47:04PM +0800, Peter Xu wrote:
> > +static inline void vtd_writel(unsigned int reg, uint32_t value)
> > +{
> > +	*(uint32_t *)(Q35_HOST_BRIDGE_IOMMU_ADDR + reg) = value;
> > +}
> > +
> > +static inline void vtd_writeq(unsigned int reg, uint64_t value)
> > +{
> > +	*(uint64_t *)(Q35_HOST_BRIDGE_IOMMU_ADDR + reg) = value;
> > +}
> > +
> > +static inline uint32_t vtd_readl(unsigned int reg)
> > +{
> > +	return *(uint32_t *)(Q35_HOST_BRIDGE_IOMMU_ADDR + reg);
> > +}
> > +
> > +static inline uint64_t vtd_readq(unsigned int reg)
> > +{
> > +	return *(uint64_t *)(Q35_HOST_BRIDGE_IOMMU_ADDR + reg);
> > +}
> 
> Is it safe to use memory dereference here?
> By contrast, Linux seems uses read*()/write*() accessors.

Looks like Linux is using memory barrior and volatile keywords to do
the protection (build_mmio_read() for x86). I think at least I can add
volatile here for each read/write to make it safer. Do you think
that'll suffice here? E.g.:

  static inline uint64_t vtd_readq(unsigned int reg)
  {
      return *(volatile uint64_t *)(Q35_HOST_BRIDGE_IOMMU_ADDR + reg);
  }

Thanks,

-- peterx