Re: [Qemu-devel] [PATCH v3 1/3] IOMMU: add VTD_CAP_CM to vIOMMU capability exposed to guest

[Peter Xu <peterx@redhat.com>]

On Mon, Jun 06, 2016 at 09:58:09PM -0600, Alex Williamson wrote:
> On Tue, 7 Jun 2016 11:20:32 +0800
> Peter Xu <peterx@redhat.com> wrote:
[...]
> > Only if cap_caching_mode() is set (which is bit 7, the CM bit), we
> > will send these invalidations. What I meant is that, we should allow
> > user to specify the CM bit, so that when we are not using VFIO
> > devices, we can skip the above flush_content() and flush_iotlb()
> > etc... So, besides the truth that we have some guests do not support
> > CM bit (like Jailhouse), performance might be another consideration
> > point that we should allow user to specify the CM bit themselfs.
> 
> I'm dubious of this, IOMMU drivers are already aware that hardware
> flushes are expensive and do batching to optimize it.  The queued
> invalidation mechanism itself is meant to allow asynchronous
> invalidations.  QEMU invalidating a virtual IOMMU might very well be
> faster than hardware.

Agree. However it seems that current Linux is still not taking this
advantage... check qi_flush_context() and qi_flush_iotlb().
qi_submit_sync() is used for both, which sends one invalidation with a
explicit wait to make sure it's sync.

> 
> > > 
> > > C: Page-walk Coherency
> > >   This field indicates if hardware access to the root, context,
> > >   extended-context and interrupt-remap tables, and second-level paging
> > >   structures for requests-without PASID, are coherent (snooped) or not.
> > >     • 0: Indicates hardware accesses to remapping structures are non-coherent.
> > >     • 1: Indicates hardware accesses to remapping structures are coherent.
> > > 
> > > Without both CM=0 and C=0, our only virtualization mechanism for
> > > maintaining a hardware cache coherent with the guest view of the iommu
> > > would be to shadow all of the VT-d structures.  For purely emulated
> > > devices, maybe we can get away with that, but I doubt the current
> > > ghashes used for the iotlb are prepared for it.  
> > 
> > Actually I haven't noticed this bit yet. I see that this will decide
> > whether guest kernel need to send specific clflush() when modifying
> > IOMMU PTEs, but shouldn't we flush the memory cache always so that we
> > can sure IOMMU can see the same memory data as CPU does?
> 
> I think it would be a question of how much the g_hash code really buys
> us in the VT-d code, it might be faster to do a lookup each time if it
> means fewer flushes.  Those hashes are useless overhead for assigned
> devices, so maybe we can avoid them when we only have assigned
> devices ;)  Thanks,

Errr, I just noticed that VFIO devices do not need emulated
cache. There are indeed lots of pending works TBD on vIOMMU side...

Thanks!

-- peterx