On Wed, 05 Apr 2023 08:35:05 +0100,
Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Tue, 04 Apr 2023 16:40:45 +0100,
> Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
> >
> > In anticipation of user hypercall filters, add the necessary plumbing to
> > get SMCCC calls out to userspace. Even though the exit structure has
> > space for KVM to pass register arguments, let's just avoid it altogether
> > and let userspace poke at the registers via KVM_GET_ONE_REG.
> >
> > This deliberately stretches the definition of a 'hypercall' to cover
> > SMCs from EL1 in addition to the HVCs we know and love. KVM doesn't
> > support EL1 calls into secure services, but now we can paint that as a
> > userspace problem and be done with it.
> >
> > Finally, we need a flag to let userspace know what conduit instruction
> > was used (i.e. SMC vs. HVC).
> >
> > Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> > ---
> > Documentation/virt/kvm/api.rst | 18 ++++++++++++++++--
> > arch/arm64/include/uapi/asm/kvm.h | 4 ++++
> > arch/arm64/kvm/handle_exit.c | 4 +++-
> > arch/arm64/kvm/hypercalls.c | 16 ++++++++++++++++
> > 4 files changed, 39 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index 9b01e3d0e757..9497792c4ee5 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -6221,11 +6221,25 @@ to the byte array.
> > __u64 flags;
> > } hypercall;
> >
> > -Unused. This was once used for 'hypercall to userspace'. To implement
> > -such functionality, use KVM_EXIT_IO (x86) or KVM_EXIT_MMIO (all except s390).
> > +
> > +It is strongly recommended that userspace use ``KVM_EXIT_IO`` (x86) or
> > +``KVM_EXIT_MMIO`` (all except s390) to implement functionality that
> > +requires a guest to interact with host userpace.
> >
> > .. note:: KVM_EXIT_IO is significantly faster than KVM_EXIT_MMIO.
> >
> > +For arm64:
> > +----------
> > +
> > +``nr`` contains the function ID of the guest's SMCCC call. Userspace is
> > +expected to use the ``KVM_GET_ONE_REG`` ioctl to retrieve the call
> > +parameters from the vCPU's GPRs.
> > +
> > +Definition of ``flags``:
> > + - ``KVM_HYPERCALL_EXIT_SMC``: Indicates that the guest used the SMC
> > + conduit to initiate the SMCCC call. If this bit is 0 then the guest
> > + used the HVC conduit for the SMCCC call.
> > +
> > ::
> >
> > /* KVM_EXIT_TPR_ACCESS */
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index f9672ef1159a..f86446c5a7e3 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -472,12 +472,16 @@ enum {
> > enum kvm_smccc_filter_action {
> > KVM_SMCCC_FILTER_HANDLE = 0,
> > KVM_SMCCC_FILTER_DENY,
> > + KVM_SMCCC_FILTER_FWD_TO_USER,
> >
> > #ifdef __KERNEL__
> > NR_SMCCC_FILTER_ACTIONS
> > #endif
> > };
> >
> > +/* arm64-specific KVM_EXIT_HYPERCALL flags */
> > +#define KVM_HYPERCALL_EXIT_SMC (1U << 0)
> > +
> > #endif
> >
> > #endif /* __ARM_KVM_H__ */
> > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > index 68f95dcd41a1..3f43e20c48b6 100644
> > --- a/arch/arm64/kvm/handle_exit.c
> > +++ b/arch/arm64/kvm/handle_exit.c
> > @@ -71,7 +71,9 @@ static int handle_smc(struct kvm_vcpu *vcpu)
> > * Trap exception, not a Secure Monitor Call exception [...]"
> > *
> > * We need to advance the PC after the trap, as it would
> > - * otherwise return to the same address...
> > + * otherwise return to the same address. Furthermore, pre-incrementing
> > + * the PC before potentially exiting to userspace maintains the same
> > + * abstraction for both SMCs and HVCs.
>
> nit: this comment really needs to find its way in the documentation so
> that a VMM author can determine the PC of the SMC/HVC. This is
> specially important for 32bit, which has a 16bit encodings for
> SMC/HVC.
>
> And thinking of it, this outlines a small flaw in this API. If
> luserspace needs to find out about the address of the HVC/SMC, it
> needs to know the *size* of the instruction. But we don't propagate
> the ESR value. I think this still works by construction (userspace can
> check PSTATE and work out whether we're in ARM or Thumb mode), but
> this feels fragile.
>
> Should we expose the ESR, or at least ESR_EL2.IL as an additional
> flag?
Just to make this a quicker round trip, I hacked the following
together. If you agree with it, I'll stick it on top and get the ball
rolling.
Thanks,
M.
>From 9b830e7a3819c2771074bebe66c1d5f20394e3cc Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@xxxxxxxxxx>
Date: Wed, 5 Apr 2023 12:48:58 +0100
Subject: [PATCH] KVM: arm64: Expose SMC/HVC width to userspace
When returning to userspace to handle a SMCCC call, we consistently
set PC to point to the instruction immediately after the HVC/SMC.
However, should userspace need to know the exact address of the
trapping instruction, it needs to know about the *size* of that
instruction. For AArch64, this is pretty easy. For AArch32, this
is a bit more funky, as Thumb has 16bit encodings for both HVC
and SMC.
Expose this to userspace with a new flag that directly derives
from ESR_EL2.IL. Also update the documentation to reflect the PC
state at the point of exit.
Finally, this fixes a small buglet where the hypercall.{args,ret}
fields would not be cleared on exit, and could contain some
random junk.
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 8 ++++++++
arch/arm64/include/uapi/asm/kvm.h | 3 ++-
arch/arm64/kvm/hypercalls.c | 16 +++++++++++-----
3 files changed, 21 insertions(+), 6 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index c8ab2f730945..103f945959ed 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6244,6 +6244,14 @@ Definition of ``flags``:
conduit to initiate the SMCCC call. If this bit is 0 then the guest
used the HVC conduit for the SMCCC call.
+ - ``KVM_HYPERCALL_EXIT_16BIT``: Indicates that the guest used a 16bit
+ instruction to initiate the SMCCC call. If this bit is 0 then the
+ guest used a 32bit instruction. An AArch64 guest always has this
+ bit set to 0.
+
+At the point of exit, PC points to the instruction immediately following
+the trapping instruction.
+
::
/* KVM_EXIT_TPR_ACCESS */
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index 3dcfa4bfdf83..b1c1edf85480 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -491,7 +491,8 @@ struct kvm_smccc_filter {
};
/* arm64-specific KVM_EXIT_HYPERCALL flags */
-#define KVM_HYPERCALL_EXIT_SMC (1U << 0)
+#define KVM_HYPERCALL_EXIT_SMC (1U << 0)
+#define KVM_HYPERCALL_EXIT_16BIT (1U << 1)
#endif
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 9a35d6d18193..3b6523f25afc 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -222,13 +222,19 @@ static void kvm_prepare_hypercall_exit(struct kvm_vcpu *vcpu, u32 func_id)
{
u8 ec = ESR_ELx_EC(kvm_vcpu_get_esr(vcpu));
struct kvm_run *run = vcpu->run;
-
- run->exit_reason = KVM_EXIT_HYPERCALL;
- run->hypercall.nr = func_id;
- run->hypercall.flags = 0;
+ u64 flags = 0;
if (ec == ESR_ELx_EC_SMC32 || ec == ESR_ELx_EC_SMC64)
- run->hypercall.flags |= KVM_HYPERCALL_EXIT_SMC;
+ flags |= KVM_HYPERCALL_EXIT_SMC;
+
+ if (!kvm_vcpu_trap_il_is32bit(vcpu))
+ flags |= KVM_HYPERCALL_EXIT_16BIT;
+
+ run->exit_reason = KVM_EXIT_HYPERCALL;
+ run->hypercall = (typeof(run->hypercall)) {
+ .nr = func_id,
+ .flags = flags,
+ };
}
int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
--
2.34.1
--
Without deviation from the norm, progress is not possible.
