On Fri, Jan 18, 2019 at 1:22 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote: > > The vCPU-run asm blob does a manual comparison of a VMCS' launched > status to execute the correct VM-Enter instruction, i.e. VMLAUNCH vs. > VMRESUME. The launched flag is a bool, which is a typedef of _Bool. > C99 does not define an exact size for _Bool, stating only that is must > be large enough to hold '0' and '1'. Most, if not all, compilers use > a single byte for _Bool, including gcc[1]. > > Originally, 'launched' was of type 'int' and so the asm blob used 'cmpl' > to check the launch status. When 'launched' was moved to be stored on a > per-VMCS basis, struct vcpu_vmx's "temporary" __launched flag was added > in order to avoid having to pass the current VMCS into the asm blob. > The new '__launched' was defined as a 'bool' and not an 'int', but the > 'cmp' instruction was not updated. > > This has not caused any known problems, likely due to compilers aligning > variables to 4-byte or 8-byte boundaries and KVM zeroing out struct > vcpu_vmx during allocation. I.e. vCPU-run accesses "junk" data, it just > happens to always be zero and so doesn't affect the result. > > [1] https://gcc.gnu.org/ml/gcc-patches/2000-10/msg01127.html > > Fixes: d462b8192368 ("KVM: VMX: Keep list of loaded VMCSs, instead of vcpus") > Cc: <stable@xxxxxxxxxxxxxxx> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
