[RFC/RFT PATCH 0/3] arm64: KVM: work around incoherency with uncached guest mappings

On Wed, Mar 04, 2015 at 03:12:12PM +0100, Andrew Jones wrote:

On Wed, Mar 04, 2015 at 01:43:02PM +0100, Ard Biesheuvel wrote:

quoted

On 4 March 2015 at 13:29, Catalin Marinas [off-list ref] wrote:

quoted

On Wed, Mar 04, 2015 at 12:50:57PM +0100, Ard Biesheuvel wrote:

quoted

I think we have established that the performance hit is not the
problem but the correctness is.

I haven't looked at the performance figures but has anyone assessed the
hit caused by doing cache maintenance in Qemu vs cacheable guest
accesses (and no maintenance)?

I'm working on a PoC of a QEMU/KVM cache maintenance approach now.
Hopefully I'll send it out this evening. Tomorrow at the latest.
Getting numbers of that approach vs. a guest's use of cached memory
for devices would take a decent amount of additional work, so won't
be part of that post.

OK.

I'm actually not sure we should care about
the numbers for a guest using normal mem attributes for device
memory - other than out of curiosity. For correctness this issue
really needs to be solved 100% host-side. We can't rely on a
guest to do different/weird things, just because it's a guest.
Ideally guests don't even know that they're guests. (Even if we
describe the memory as cache-able to the guest, I don't think we
can rely on the guest believing us.)

I disagree it is 100% a host-side issue. It is a host-side issue _if_
the host tells the guest that the (virtual) device is non-coherent (or,
more precisely, it does not explicitly tell the guest that the device is
coherent). If the guest thinks the (virtual) device is non-coherent
because of information passed by the host, I fully agree that the host
needs to manage the cache coherency.

However, the host could also pass a "dma-coherent" property in the DT
given to the guest and avoid any form of cache maintenance. If the guest
does not honour such coherency property, it's a guest problem and it
needs fixing in the guest. This isn't any different from a real physical
device behaviour.

(there are counter arguments for the latter as well like emulating old
platforms that never had coherency but from a performance/production
perspective, I strongly recommend that guests are passed the
"dma-coherent" property for such virtual devices)

-- 
Catalin