On Fri, Mar 24, 2017 at 02:56:51PM +0000, Marc Zyngier wrote: > 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? > I'm perfectly happy with documenting the feature and documenting it here. It just felt arbitrary to me when I read the code that we could just step on x5/x6. Thanks, -Christoffer
