Re: [PATCH v3 08/13] KVM: arm64: Add support for KVM_EXIT_HYPERCALL

[Marc Zyngier <maz@kernel.org>]

On Wed, 05 Apr 2023 08:35:05 +0100,
Marc Zyngier <maz@kernel.org> wrote:
> 
> On Tue, 04 Apr 2023 16:40:45 +0100,
> Oliver Upton <oliver.upton@linux.dev> 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@linux.dev>
> > ---
> >  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@kernel.org>
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@kernel.org>
---
 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.