On 24/03/17 14:33, Christoffer Dall wrote: > On Tue, Mar 21, 2017 at 07:20:37PM +0000, Marc Zyngier wrote: >> We now have a full hyp-stub implementation in the KVM init code, >> but the main KVM code only supports HVC_GET_VECTORS, which is not >> enough. >> >> Instead of reinventing the wheel, let's reuse the init implementation >> by branching to the idmap page when called with a hyp-stub hypercall. >> >> Reviewed-by: James Morse <james.morse@xxxxxxx> >> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> >> --- >> arch/arm64/kvm/hyp/hyp-entry.S | 24 +++++++++++++++++++----- >> 1 file changed, 19 insertions(+), 5 deletions(-) >> >> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S >> index d8ef788646c6..4f34c5996f86 100644 >> --- a/arch/arm64/kvm/hyp/hyp-entry.S >> +++ b/arch/arm64/kvm/hyp/hyp-entry.S >> @@ -87,10 +87,24 @@ alternative_endif >> /* Here, we're pretty sure the host called HVC. */ >> ldp x0, x1, [sp], #16 >> >> - cmp x0, #HVC_GET_VECTORS >> - b.ne 1f >> - mrs x0, vbar_el2 >> - b 2f >> + /* Check for a stub HVC call */ >> + cmp x0, #HVC_STUB_HCALL_NR >> + b.hs 1f >> + >> + /* >> + * Compute the idmap address of __kvm_handle_stub_hvc and >> + * jump there. Since we use kimage_voffset, do not use the >> + * HYP VA for __kvm_handle_stub_hvc, but the kernel VA instead >> + * (by loading it from the constant pool). >> + * >> + * Preserve x0-x4, which may contain stub parameters. >> + */ >> + ldr x5, =__kvm_handle_stub_hvc >> + ldr_l x6, kimage_voffset > > Isn't it a bit dodgy to just overwrite x5 and x6 in something which is > not a function? I know that in practice this always gets called through > a function call and we can rely on the calling convention, but this can > break if you issue a hypercall to KVM's HVC sub implementation using > inline assembly, I think. > > Am I missing something here? I don't think you're missing anything. We're definitely relying on getting there via a function call which is going to preserve the caller-saved registers. The only case where we issue a naked HVC call is when we use HVC_SOFT_RESTART. At this point, corrupted registers is not much of a worry. Now, I see two ways of dealing with this: - We save x5/x6 on the stack, restoring them in the stub handler , right before the eret - Or we simply document the behaviour in asm/virt.h so that people adding new stub methods know the restrictions. Thoughts? M. -- Jazz is not dead. It just smells funny...
