linux-kernel - Re: [PATCH v3 1/2] KVM: arm64: Support FFA_MSG_SEND_DIRECT

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Message-ID: <aWDoeMRZbbsQ2htP@willie-the-truck>
Date: Fri, 9 Jan 2026 11:37:28 +0000
From: Will Deacon <will@...nel.org>
To: Sebastian Ene <sebastianene@...gle.com>
Cc: perlarsen@...gle.com, Marc Zyngier <maz@...nel.org>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Yeoreum Yun <yeoreum.yun@....com>, Ben Horgan <ben.horgan@....com>,
	Oliver Upton <oupton@...nel.org>,
	Armelle Laine <armellel@...gle.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] KVM: arm64: Support FFA_MSG_SEND_DIRECT_REQ in
 host handler

Hey Seb,

Cheers for the reply.

On Fri, Jan 09, 2026 at 11:18:33AM +0000, Sebastian Ene wrote:
> On Thu, Jan 08, 2026 at 03:26:21PM +0000, Will Deacon wrote:
> > On Wed, Nov 19, 2025 at 02:07:53AM +0000, Per Larsen via B4 Relay wrote:
> > > From: Sebastian Ene <sebastianene@...gle.com>
> > > 
> > > Allow direct messages to be forwarded from the host. The host should
> > > not be sending framework messages so they are filtered out.
> > > 
> > > Signed-off-by: Sebastian Ene <sebastianene@...gle.com>
> > > Reviewed-by: Yeoreum Yun <yeoreum.yun@....com>
> > > Signed-off-by: Per Larsen <perlarsen@...gle.com>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/ffa.c | 22 ++++++++++++++++++++++
> > >  include/linux/arm_ffa.h       |  3 +++
> > >  2 files changed, 25 insertions(+)
> > > 
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > index 58b7d0c477d7fce235fc70d089d175c7879861b5..a38a3ab497e5eac11777109684a33f02d88d09a1 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > @@ -862,6 +862,23 @@ static void do_ffa_part_get(struct arm_smccc_1_2_regs *res,
> > >  	hyp_spin_unlock(&host_buffers.lock);
> > >  }
> > >  
> > > +static void do_ffa_direct_msg(struct arm_smccc_1_2_regs *res,
> > > +			      struct kvm_cpu_context *ctxt,
> > > +			      u64 vm_handle)
> > > +{
> > > +	DECLARE_REG(u32, flags, ctxt, 2);
> > > +
> > > +	struct arm_smccc_1_2_regs *args = (void *)&ctxt->regs.regs[0];
> > > +
> > > +	/* filter out framework messages */
> > > +	if (FIELD_GET(FFA_MSG_FLAGS_MSG_TYPE, flags)) {
> > 
> > Wouldn't we be better off just checking that flags is 0? The rest of it
> > is SBZ or MBZ in the current spec.
> 
> Yes, we can simplify it in this way.

I think it would also be more robust if new messaging types are added
in future, as we would fail safe.

> > > +		do_ffa_direct_msg(&res, host_ctxt, HOST_FFA_ID);
> > 
> > What's the point of passing HOST_FFA_ID here? Is that supposed to end up
> > in the Sender ID bits of W1?
> 
> I can remove it, this doesn't bring too much for upstream but on the
> android kernel with guest-ffa it makes sense because we need to validate
> the sender to prevent impersonation.

We could also validate that the sender is HOST_FFA_ID in this case, but
that seems to be missing atm.

Cheers,

Will