Re: [Xen-devel] [PATCH V2 11/25] x86/hvm: Introduce a emulated VTD for HVM

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Aug 24, 2017 at 10:16:32AM +0800, Lan Tianyu wrote:
> On 2017年08月23日 15:58, Roger Pau Monné wrote:
> > On Wed, Aug 09, 2017 at 04:34:12PM -0400, Lan Tianyu wrote:
> >> From: Chao Gao <chao.gao@xxxxxxxxx>
> >> +}
> >> +
> >> +#define vvtd_get_reg_quad(vvtd, reg, val) do {  \
> >> +    (val) = vvtd_get_reg(vvtd, (reg) + 4 );     \
> >> +    (val) = (val) << 32;                        \
> >> +    (val) += vvtd_get_reg(vvtd, reg);           \
> >> +} while(0)
> >> +#define vvtd_set_reg_quad(vvtd, reg, val) do {  \
> >> +    vvtd_set_reg(vvtd, reg, (val));             \
> >> +    vvtd_set_reg(vvtd, (reg) + 4, (val) >> 32); \
> >> +} while(0)
> > 
> > You seem to need to access hvm_hw_vvtd_regs using different sizes, why
> > not do:
> > 
> > union hvm_hw_vvtd_regs {
> >     uint8_t  data8[1024];
> >     uint16_t data16[512];
> >     uint32_t data32[256];
> >     uint64_t data64[128];
> > };
> > 
> > Then the access is much more straightforward and you don't need the
> > complicated helpers that you have above.
> 
> Yes, that will be simpler.

Keep in mind (as said in another patch) that this approach will only
work correctly as long as you force accesses to be size aligned, which
you where not doing now.

I've looked at the VT-d spec, but I cannot find any section that
explains the restrictions on access sizes and alignments.

Roger.

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