On Thu, Jan 11, 2024 at 5:25 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > It is unusual to assign a huge RAM to guest, but passthrough a device also may trigger > > this issue which we have met, i.e. alloc memslot for the 64bit BAR which can set > > bits[51:48]. BIOS can control the BAR address, e.g. seabios moved 64bit pci window > > to end of address space by using advertised physical bits[1]. > > Drat. Do you know if these CPUs are going to be productized? We'll still need > something in KVM either way, but whether or not the problems are more or less > limited to funky software setups might influence how we address this. Wait, we do have an API for guest physical address size. It's KVM_GET_SUPPORTED_CPUID2: the # of bits is in leaf 0x80000008, bits 0:7 of EAX. In fact that leaf is what firmware uses to place the BARs. So it just needs to be adjusted for VMX in __do_cpuid_func, and looked up in selftests. Paolo
