On Mon, Feb 26, 2024 at 09:30:33AM +0800, Xiaoyao Li wrote: > On 2/23/2024 9:35 AM, Sean Christopherson wrote: > > On Tue, 09 Jan 2024 16:23:40 -0800, Sean Christopherson wrote: > > > Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query > > > whether or not the CPU supports 5-level EPT paging. EPT capabilities are > > > enumerated via MSR, i.e. aren't accessible to userspace without help from > > > the kernel, and knowing whether or not 5-level EPT is supported is sadly > > > necessary for userspace to correctly configure KVM VMs. > > > > > > When EPT is enabled, bits 51:49 of guest physical addresses are consumed > > > if and only if 5-level EPT is enabled. For CPUs with MAXPHYADDR > 48, KVM > > > *can't* map all legal guest memory if 5-level EPT is unsupported, e.g. > > > creating a VM with RAM (or anything that gets stuffed into KVM's memslots) > > > above bit 48 will be completely broken. > > > > > > [...] > > > > Applied to kvm-x86 vmx, with a massaged changelog to avoid presenting this as a > > bug fix (and finally fixed the 51:49=>51:48 goof): > > > > Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query > > whether or not the CPU supports 5-level EPT paging. EPT capabilities are > > enumerated via MSR, i.e. aren't accessible to userspace without help from > > the kernel, and knowing whether or not 5-level EPT is supported is useful > > for debug, triage, testing, etc. > > For example, when EPT is enabled, bits 51:48 of guest physical addresses > > are consumed by the CPU if and only if 5-level EPT is enabled. For CPUs > > with MAXPHYADDR > 48, KVM *can't* map all legal guest memory if 5-level > > EPT is unsupported, making it more or less necessary to know whether or > > not 5-level EPT is supported. > > > > [1/1] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace > > https://github.com/kvm-x86/linux/commit/b1a3c366cbc7 > > Do we need a new KVM CAP for this? This decides how to interact with old > kernel without this patch. In that case, no ept_5level in /proc/cpuinfo, > what should we do in the absence of ept_5level? treat it only 4 level EPT > supported? Maybe also adding flag for 4-level EPT can be an option. If userspace checks both 4-level and 5-level are not in /proc/cpuinfo, it can regard the kernel as old. Thanks, Tao > > > > > -- > > https://github.com/kvm-x86/linux/tree/next > > >
