Re: [Qemu-devel] [PATCH] Add a memory barrier to guest memory access functions

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 21, 2012 at 07:16:27PM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2012-05-21 at 19:07 +1000, Benjamin Herrenschmidt wrote:
> 
> > One thing that might alleviate some of your concerns would possibly be
> > to "remember" in a global (to be replaced with a thread var eventually)
> > the last transfer direction and use a simple test to chose the barrier,
> > ie, store + store -> wmb, load + load -> rmb, other -> mb.

But how do you know guest did a store?

> > 
> > But first I'd be curious if some x86 folks could actually measure the
> > impact of the patch as I proposed it. That would give us an idea of how
> > bad the performance problem is and how far we need to go to address it.
> 
> Another option.... go back to something more like the original patch,
> ie, put the barrier in the new dma_* accessors (and provide a
> non-barrier one while at it) rather than the low level cpu_physical_*
> accessor.
> 
> That makes it a lot easier for selected driver to be converted to avoid
> the barrier in thing like code running in the vcpu context. It also
> means that virtio doesn't get any added barriers which is what we want
> as well.
> 
> IE. Have something along the lines (based on the accessors added by the
> iommu series) (using __ kernel-style, feel free to provide a better
> naming)
> 
> static inline int __dma_memory_rw( ... args ... )
> {
>     if (!dma_has_iommu(dma)) {
>         /* Fast-path for no IOMMU */
>         cpu_physical_memory_rw( ... args ...);
>         return 0;
>     } else {
>         return iommu_dma_memory_rw( ... args ...);
>     }
> }
> 
> static inline int dma_memory_rw( ... args ... )
> {
> 	smp_mb(); /* Or use finer grained as discussied earlier */
> 
> 	return __dma_memory_rw( ... args ... )

Heh. But don't we need an mb afterwards too?

> }
> 
> And corresponding __dma_memory_read/__dma_memory_write (again, feel
> free to suggest a more "qemu'ish" naming if you don't like __, it's
> a kernel habit, not sure what you guys do in qemu land).
> 
> Cheers,
> Ben.

And my preference is to first convert everyone to __ variants and
carefully switch devices to the barrier version after a bit of
consideration.

-- 
MST