Hi Marc,
On Thu, Oct 12, 2017 at 04:49:44PM +0100, Marc Zyngier wrote:
> On 12/10/17 11:41, Christoffer Dall wrote:
> > We already have the percpu area for the host cpu state, which points to
> > the VCPU, so there's no need to store the VCPU pointer on the stack on
> > every context switch. We can be a little more clever and just use
> > tpidr_el2 for the percpu offset and load the VCPU pointer from the host
> > context.
> >
> > This requires us to have a scratch register though, so we take the
> > chance to rearrange some of the el1_sync code to only look at the
> > vttbr_el2 to determine if this is a trap from the guest or an HVC from
> > the host. We do add an extra check to call the panic code if the kernel
> > is configured with debugging enabled and we saw a trap from the host
> > which wasn't an HVC, indicating that we left some EL2 trap configured by
> > mistake.
> >
> > Signed-off-by: Christoffer Dall <christoffer.d...@linaro.org>
> > ---
> > arch/arm64/include/asm/kvm_asm.h | 20 ++++++++++++++++++++
> > arch/arm64/kernel/asm-offsets.c | 1 +
> > arch/arm64/kvm/hyp/entry.S | 5 +----
> > arch/arm64/kvm/hyp/hyp-entry.S | 39
> > ++++++++++++++++++---------------------
> > arch/arm64/kvm/hyp/switch.c | 2 +-
> > 5 files changed, 41 insertions(+), 26 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_asm.h
> > b/arch/arm64/include/asm/kvm_asm.h
> > index ab4d0a9..7e48a39 100644
> > --- a/arch/arm64/include/asm/kvm_asm.h
> > +++ b/arch/arm64/include/asm/kvm_asm.h
> > @@ -70,4 +70,24 @@ extern u32 __init_stage2_translation(void);
> >
> > #endif
> >
> > +#ifdef __ASSEMBLY__
> > +.macro get_host_ctxt reg, tmp
> > + /*
> > + * '=kvm_host_cpu_state' is a host VA from the constant pool, it may
> > + * not be accessible by this address from EL2, hyp_panic() converts
> > + * it with kern_hyp_va() before use.
> > + */
>
> This really looks like a stale comment, as there is no hyp_panic
> involved here anymore (thankfully!).
>
> > + ldr \reg, =kvm_host_cpu_state
> > + mrs \tmp, tpidr_el2
> > + add \reg, \reg, \tmp
> > + kern_hyp_va \reg
>
> Here, we're trading a load from the stack for a load from the constant
> pool. Can't we do something like:
>
> adr_l \reg, kvm_host_cpu_state
> msr \tmp, tpidr_el2
> add \reg, \reg, \tmp
>
> and that's it? This relies on the property that the kernel/hyp offset is
> constant, and that it doesn't matter if we add the offset to a kernel VA
> or a HYP VA... Completely untested of course!
>
Coming back to this one, annoyingly, it doesn't seem to work. This is
the code I use for get_host_ctxt:
.macro get_host_ctxt reg, tmp
adr_l \reg, kvm_host_cpu_state
mrs \tmp, tpidr_el2
add \reg, \reg, \tmp
kern_hyp_va \reg
.endm
And this is the disassembly for one of the uses in the hyp code:
adrp x0, ffff000008ca9000 <overflow_stack+0xd20>
add x0, x0, #0x7f0
mrs x1, tpidr_el2
add x0, x0, x1
and x0, x0, #0xffffffffffff
For comparison, the following C-code:
struct kvm_cpu_context *host_ctxt;
host_ctxt = this_cpu_ptr(&kvm_host_cpu_state);
host_ctxt = kern_hyp_va(host_ctxt);
Gets compiled into this:
adrp x0, ffff000008ca9000 <overflow_stack+0xd20>
add x0, x0, #0x7d0
mrs x1, tpidr_el1
add x0, x0, #0x20
add x0, x0, x1
and x0, x0, #0xffffffffffff
And, during hyp init we do:
mrs x1, tpidr_el1
msr tpidr_el2, x1
Any ideas what could be going on here?
Thanks,
-Christoffer
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