2016-04-15 12:48+0200, Paolo Bonzini: > On 14/04/2016 18:29, Radim Krčmář wrote: >> I don't see that as a compromise. igd would fail even if we fixed the >> host side, so we'll have problems regardless of what we do. > > Would it? I suppose that Shuai tested his patch. I meant a fix that would make KVM behave according to the spec, which would not help igd on kvm64 CPU model, because igd accesses MSRs that don't exist, so #GP is the only response. The patch does completely work around current igd issue, but we trade an obvious #GP for an unknown error when the guest acts as if MSR_PLATFORM_INFO was 0. >> We have a bug, because certain v/f/m/s implies some features (MSRs, >> constant_tsc, ...) and those aren't emulated. >> >> I do agree that we don't want to fix the bug, either by whitelisting and >> emulating features that makes little sense in virt or by forcing guests >> to adopt new v/f/m/s (the latter option is more reasonable), > > Well, the Pentium was the last processor without MSRs. :) More code > would break if you set f=5 than if you return a bogus value for > MSR_PLATFORM_INFO. True. The loss becomes less clear with f=15 (kvm64 and Pentium 4) that "solves" the igd bug by not providing MSR_PLATFORM_INFO ... Btw. do we emulate any feature of newer GenuineIntel/f/m/s? > This is the compromise I was referring to. Ah, thanks. I only saw a complete disadvantage for KVM. :) > The only solution is to bug Intel to add CPUID bits even for > non-architectural features. Then _if_ the CPUID bit is there you use > f/m/s to find the details of the feature. Intel likes to get feedback > from us and we did provide such feedback. The problem is that the 2-3 > years that pass between giving feedback and getting our hands on the > silicon. That is great! (CPUID-only feature detection would suit us more, but f/m/s will at least have a chance to regain trust under KVM.) -- 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
