• Mohammed Gamal's avatar
    KVM: x86: Add a capability for GUEST_MAXPHYADDR < HOST_MAXPHYADDR support · 3edd6839
    Mohammed Gamal authored
    This patch adds a new capability KVM_CAP_SMALLER_MAXPHYADDR which
    allows userspace to query if the underlying architecture would
    support GUEST_MAXPHYADDR < HOST_MAXPHYADDR and hence act accordingly
    (e.g. qemu can decide if it should warn for -cpu ..,phys-bits=X)
    
    The complications in this patch are due to unexpected (but documented)
    behaviour we see with NPF vmexit handling in AMD processor.  If
    SVM is modified to add guest physical address checks in the NPF
    and guest #PF paths, we see the followning error multiple times in
    the 'access' test in kvm-unit-tests:
    
                test pte.p pte.36 pde.p: FAIL: pte 2000021 expected 2000001
                Dump mapping: address: 0x123400000000
                ------L4: 24c3027
                ------L3: 24c4027
                ------L2: 24c5021
                ------L1: 1002000021
    
    This is because the PTE's accessed bit is set by the CPU hardware before
    the NPF vmexit. This is handled completely by hardware and cannot be fixed
    in software.
    
    Therefore, availability of the new capability depends on a boolean variable
    allow_smaller_maxphyaddr which is set individually by VMX and SVM init
    routines. On VMX it's always set to true, on SVM it's only set to true
    when NPT is not enabled.
    
    CC: Tom Lendacky <thomas.lendacky@amd.com>
    CC: Babu Moger <babu.moger@amd.com>
    Signed-off-by: default avatarMohammed Gamal <mgamal@redhat.com>
    Message-Id: <20200710154811.418214-10-mgamal@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    3edd6839
x86.c 281 KB