On Wed, Apr 24, 2019 at 09:59:50AM +0800, Like Xu wrote: > On 2019/4/24 1:44, Sean Christopherson wrote: > >Right, but isn't the f_intel_pt check for example completely irrelevant? > >f_intel_pt is true if and only if hardware supports PT, i.e. CPUID.0.EAX > >and thus entry->eax will already be >=0x14. > > The f_intel_pt check is not only about hardware supports check but also > module_param (pt_mode) supports check. > > So the case is the host does have PT support which means (host CPUID.0.EAX > already be >=0x14 for Intel CPUs) but kvm doesn't want advertise it and thus > the min() operation is needed. > > > > >I don't fully understand whether or not KVM needs to raise the minimum to > >0xb regardless of h/w XSAVE support, but it's likely irrelevant in the end. > > > >Anyways, back to 0x1f, kvm_supported_intel_mcp() returns true if and only > >if hardware's CPUID.0.EAX >= 0x1f, > > According to latest SDM, the max hardware CPUID.0.EAX is 0x1f and BIOS would > expose 0x1f only for multi-chip packaging CPUs (at least for now). > > >i.e. adjusting entry->eax is always a > >nop. So if KVM wants to advertise leaf 0x1f only when it's supported in > >hardware then adjusting entry->eax is unnecessary, and if KVM wants to > >unconditionally advertise 0x1f then adjusting entry->eax should also be > >done unconditionally. > > It we have no check on kvm_supported_intel_mcp() in legacy code, > CPUID.0.EAX would be min() and thus less than 0x1f which means the cpuid.1f > info is not exposed. Ah crud, I'm an idiot. I just spent two days conflating min() and max(). So yeah, everything makes total sense now. My apologies for wasting your time, I'll re-review the patch. > > I know your point is to avoid min() totally (I thought so at the time) and I > have pointed out it's necessary for kvm features setting. > > If KVM wants to unconditionally advertise 0x1f (in EMULATED way), > kvm needs cover other side effects and this patch only advertises 0x1f > when hardware has it. > > It's very common that guest wants to set 0x1f regardless of h/w support > and this is another story. > > > > >>>Given that the original code > >>>was "entry->eax = min(entry->eax, (u32)0xb);", my *guess* is that the > >>>idea was to always report "Extended Topology Enumeration Leaf" as > >>>supported so that userspace can enumerate the VM's topology to the guest > >>>even when hardware itself doesn't do so. > >> > >>If the host cpu mode is too antiquated to support 0xb, it wouldn't report > >>0xb for sure. The host cpuid.0.eax has been over 0xb for a long time and > >>reached 0x1f in the latest SDM. > >> > >>AFAICT, the original code keeps minimum cpuid.0.eax out of features guest > >>just used or at least it claimed to use. > >> > >>> > >>>Assuming we want to allow userspace to use "V2 Extended Topology > >>>Enumeration Leaf" regardless of hardware support, then this can simply be: > >>> > >>> entry->eax = min(entry->eax, (u32)0x1f); > >>> > >>>Or am I completely missing something? > > >
