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: > > On 4 March 2015 at 13:29, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > On Wed, Mar 04, 2015 at 12:50:57PM +0100, Ard Biesheuvel wrote: > > >> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
