On 8/31/2022 8:50 PM, Gerd Hoffmann wrote:
When the guest (firmware specifically) knows how big the address space actually is it can be used better. Some more background: https://bugzilla.redhat.com/show_bug.cgi?id=2084533
QEMU enables host-phys-bits for "-cpu host/max" in host_cpu_max_instance_init();
I think the problem is for all the named CPU model, that they don't have phys_bits defined. Thus they all have "cpu->phys-bits == 0", which leads to cpu->phys_bits = TCG_PHYS_ADDR_BITS (36 for 32-bits build and 40 for 64-bits build)
Anyway, IMO, guest including guest firmware, should always consult from CPUID leaf 0x80000008 for physical address length. Tt is the duty of userspace VMM, here QEMU, to ensure VM's host physical address length not exceeding host's. If userspace VMM cannot ensure this, guest is likely hitting problem.
This is a RfC series exposes the information via cpuid. take care, Gerd Gerd Hoffmann (2): [hack] reserve bit KVM_HINTS_HOST_PHYS_BITS [RfC] expose host-phys-bits to guest include/standard-headers/asm-x86/kvm_para.h | 3 ++- target/i386/cpu.h | 3 --- hw/i386/microvm.c | 6 +++++- target/i386/cpu.c | 3 +-- target/i386/host-cpu.c | 4 +++- target/i386/kvm/kvm.c | 1 + 6 files changed, 12 insertions(+), 8 deletions(-)
