• Christoffer Dall's avatar
    KVM: ARM: World-switch implementation · f7ed45be
    Christoffer Dall authored
    Provides complete world-switch implementation to switch to other guests
    running in non-secure modes. Includes Hyp exception handlers that
    capture necessary exception information and stores the information on
    the VCPU and KVM structures.
    
    The following Hyp-ABI is also documented in the code:
    
    Hyp-ABI: Calling HYP-mode functions from host (in SVC mode):
       Switching to Hyp mode is done through a simple HVC #0 instruction. The
       exception vector code will check that the HVC comes from VMID==0 and if
       so will push the necessary state (SPSR, lr_usr) on the Hyp stack.
       - r0 contains a pointer to a HYP function
       - r1, r2, and r3 contain arguments to the above function.
       - The HYP function will be called with its arguments in r0, r1 and r2.
       On HYP function return, we return directly to SVC.
    
    A call to a function executing in Hyp mode is performed like the following:
    
            <svc code>
            ldr     r0, =BSYM(my_hyp_fn)
            ldr     r1, =my_param
            hvc #0  ; Call my_hyp_fn(my_param) from HYP mode
            <svc code>
    
    Otherwise, the world-switch is pretty straight-forward. All state that
    can be modified by the guest is first backed up on the Hyp stack and the
    VCPU values is loaded onto the hardware. State, which is not loaded, but
    theoretically modifiable by the guest is protected through the
    virtualiation features to generate a trap and cause software emulation.
    Upon guest returns, all state is restored from hardware onto the VCPU
    struct and the original state is restored from the Hyp-stack onto the
    hardware.
    
    SMP support using the VMPIDR calculated on the basis of the host MPIDR
    and overriding the low bits with KVM vcpu_id contributed by Marc Zyngier.
    
    Reuse of VMIDs has been implemented by Antonios Motakis and adapated from
    a separate patch into the appropriate patches introducing the
    functionality. Note that the VMIDs are stored per VM as required by the ARM
    architecture reference manual.
    
    To support VFP/NEON we trap those instructions using the HPCTR. When
    we trap, we switch the FPU.  After a guest exit, the VFP state is
    returned to the host.  When disabling access to floating point
    instructions, we also mask FPEXC_EN in order to avoid the guest
    receiving Undefined instruction exceptions before we have a chance to
    switch back the floating point state.  We are reusing vfp_hard_struct,
    so we depend on VFPv3 being enabled in the host kernel, if not, we still
    trap cp10 and cp11 in order to inject an undefined instruction exception
    whenever the guest tries to use VFP/NEON. VFP/NEON developed by
    Antionios Motakis and Rusty Russell.
    
    Aborts that are permission faults, and not stage-1 page table walk, do
    not report the faulting address in the HPFAR.  We have to resolve the
    IPA, and store it just like the HPFAR register on the VCPU struct. If
    the IPA cannot be resolved, it means another CPU is playing with the
    page tables, and we simply restart the guest.  This quirk was fixed by
    Marc Zyngier.
    Reviewed-by: default avatarWill Deacon <will.deacon@arm.com>
    Reviewed-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
    Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
    Signed-off-by: default avatarAntonios Motakis <a.motakis@virtualopensystems.com>
    Signed-off-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Signed-off-by: default avatarChristoffer Dall <c.dall@virtualopensystems.com>
    f7ed45be
arm.c 18.1 KB