Re: [PATCH 01/11] KVM: arm64: Store vcpu on the stack during __guest_enter()

Christoffer Dall
2017-08-09T08:48:09Z

On Tue, Aug 08, 2017 at 05:48:29PM +0100, James Morse wrote:
> Hi Christoffer,
>
> On 06/06/17 20:59, Christoffer Dall wrote:
> > On Mon, May 15, 2017 at 06:43:49PM +0100, James Morse wrote:
> >> KVM uses tpidr_el2 as its private vcpu register, which makes sense for
> >> non-vhe world switch as only KVM can access this register. This means
> >> vhe Linux has to use tpidr_el1, which KVM has to save/restore as part
> >> of the host context.
> >>
> >> __guest_enter() stores the host_ctxt on the stack, do the same with
> >> the vcpu.
>
> >> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> >> index 12ee62d6d410..113735df7d01 100644
> >> --- a/arch/arm64/kvm/hyp/entry.S
> >> +++ b/arch/arm64/kvm/hyp/entry.S
> >> @@ -159,9 +159,15 @@ abort_guest_exit_end:
> >> ENDPROC(__guest_exit)
> >>
> >> ENTRY(__fpsimd_guest_restore)
> >> + // x0: esr
> >> + // x1: vcpu
> >> + // x2-x29,lr: vcpu regs
> >> + // vcpu x0-x1 on the stack
> >> stp x2, x3, [sp, #-16]!
> >> stp x4, lr, [sp, #-16]!
> >>
> >> + mov x3, x1
> >> +
> >
> > nit: can you avoid this by using x1 for the vcpu pointer in this routine
> > instead?
>
> Unfortunately x1 is clobbered by the __fpsimd_{save,restore}_state() macros that
> are called further down this function.
>
> (its a bit obscure:
> > fpsimd_save x0, 1
> that '1' is used to generate 'x1' or 'w1' in includes/asm/fpsimdmacros.h)
>

Ah, I guess I missed that.

Thanks,
-Christoffer