2017-08-17 01:31+0300, Michael S. Tsirkin: > On Wed, Aug 16, 2017 at 11:25:35PM +0200, Paolo Bonzini wrote: > > On 16/08/2017 21:59, Michael S. Tsirkin wrote: > > > On Wed, Aug 16, 2017 at 09:03:17PM +0200, Radim Krčmář wrote: > > >>>> how about we blacklist nested virt for this optimization? > > >> > > >> Not every hypervisor can be easily detected ... > > > > > > Hypervisors that don't set a hypervisor bit in CPUID are violating the > > > spec themselves, aren't they? Anyway, we can add a management option > > > for use in a nested scenario. > > > > No, the hypervisor bit only says that CPUID leaf 0x40000000 is defined. > > See for example > > https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1009458: > > "Intel and AMD have also reserved CPUID leaves 0x40000000 - 0x400000FF > > for software use. Hypervisors can use these leaves to provide an > > interface to pass information from the hypervisor to the guest operating > > system running inside a virtual machine. The hypervisor bit indicates > > the presence of a hypervisor and that it is safe to test these > > additional software leaves". > > Looks like it's not a bug then. Still, most hypervisors do have this > leaf so it's a reasonable way that will catch most issues. We can > always blacklist more as they are found. Additionally let's go ahead > and add ability for userspace to disable fast MMIO for these > hypervisors we failed to detect. In the worst case, I'd make faster mmio an opt-in unsafe feature regardless of what we run on. Users that just want KVM to work get the default and people who care about utmost performance can jump through loops. > > >> KVM uses standard features and SDM clearly says that the > > >> instruction length field is undefined. > > > > > > True. Let's see whether intel can commit to a stronger definition. > > > I don't think there's any rush to make this change. > > > > I disagree. Relying on undefined processor features is a bad idea. > > Maybe it was a bad idea 3 years ago, yes. In 2012 I posted "kvm_para: > add mmio word store hypercall" as an alternative. Was nacked as MMIO > was seen as safer and better. By now many people rely on mmio being > fast. Let's talk to hardware guys to define the feature before we give > up and spend years designing an alternative. The change is not backward-compatible wrt. SDM, but all processors might actually be behaving like we want ... (I'd assert undefined behavior add a vm-exit flag if I were to allow it, though.) > > > It's just that this has been there for 3 years and people have built a > > > product around this. > > > > Around 700 clock cycles? > > > > Paolo > > About 30% the cost of exit, isn't it? There are definitely workloads > where cost of exit gates performance. We didn't work on fast mmio based > on theoretical assumptions. But maybe I am wrong. We'll see. Jason here > volunteered to test your patch and we'll see what comes out of it. If > I'm wrong and it's about 1%, I won't split hairs. I'm ok with waiting for the numbers as I hope that we won't have to resort to adding special cases.
