Re: [Xen-devel] [PATCH] vt-d: use two 32-bit writes to update DMAR fault address registers

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 09/11/17 10:38 +0100, Roger Pau Monné wrote:
> On Mon, Sep 11, 2017 at 02:00:48PM +0800, Haozhong Zhang wrote:
> > The 64-bit DMAR fault address is composed of two 32 bits registers
> > DMAR_FEADDR_REG and DMAR_FEUADDR_REG. According to VT-d spec:
> > "Software is expected to access 32-bit registers as aligned doublewords",
> > a hypervisor should use two 32-bit writes to DMAR_FEADDR_REG and
> > DMAR_FEUADDR_REG separately in order to update a 64-bit fault address,
> > rather than a 64-bit write to DMAR_FEADDR_REG.
> > 
> > Though I haven't seen any errors caused by such one 64-bit write on
> > real machines, it's still better to follow the specification.
> 
> Either the patch description is missing something or the patch is
> wrong. You should mention why is the write to the high part of the
> address now conditional on x2APIC being enabled, when it didn't use to
> be before.
>

When x2APIC is disabled, DMAR_FEUADDR_REG is reserved and it's not
necessary to update it. The original code always writes zero to it in
that case, which is also correct.

Haozhong

> [...]
> > -    dmar_writeq(iommu->reg, DMAR_FEADDR_REG, msg.address);
> > +    dmar_writel(iommu->reg, DMAR_FEADDR_REG, msg.address_lo);
> > +    if (x2apic_enabled)
> > +        dmar_writel(iommu->reg, DMAR_FEUADDR_REG, msg.address_hi);
> >      spin_unlock_irqrestore(&iommu->register_lock, flags);
> 
> Thanks, Roger.

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