linux-kernel - Re: [PATCH v2 4/4] KVM: arm64: Release the ownership of the hyp rx buffer to Trustzone

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Message-ID: <Z8ltm55D1jrv8QtX@bogus>
Date: Thu, 6 Mar 2025 09:40:43 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Will Deacon <will@...nel.org>
Cc: Sebastian Ene <sebastianene@...gle.com>, catalin.marinas@....com,
	Sudeep Holla <sudeep.holla@....com>, joey.gouly@....com,
	maz@...nel.org, oliver.upton@...ux.dev, snehalreddy@...gle.com,
	suzuki.poulose@....com, vdonnefort@...gle.com, yuzenghui@...wei.com,
	kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kernel-team@...roid.com,
	Andrei Homescu <ahomescu@...gle.com>
Subject: Re: [PATCH v2 4/4] KVM: arm64: Release the ownership of the hyp rx
 buffer to Trustzone

On Wed, Mar 05, 2025 at 07:34:26PM +0000, Will Deacon wrote:
> On Wed, Mar 05, 2025 at 09:41:04AM +0000, Sudeep Holla wrote:
> > On Wed, Mar 05, 2025 at 12:45:23AM +0000, Will Deacon wrote:
> > > Hmm, the FFA spec is characteristically unclear as to whether or not we
> > > need to release the rx buffer in the case that the flags indicate use of
> > > the rx buffer but the returned partition count is 0.
> > > 
> > > Sudeep -- do you know what we should be doing in that case?
> > > 
> > 
> > We need to call RX_RELEASE here. I went back to the spec to confirm the
> > same again.
> > 
> > v1.2 EAC0 spec Section 7.2.2.4.2 Transfer of buffer ownership
> > (Or just look for the section title in any version of the spec)
> > "
> > 2. Ownership transfer for the RX buffer takes place as follows.
> >     2. For a framework message,
> >        1. Completion of the FFA_PARTITION_INFO_GET ABI transfers the ownership
> >        of the caller’s RX buffer from the Producer to the Consumer.
> > 3. For both types of messages, an invocation of the following FF-A ABIs
> >     transfers the ownership from the Consumer to the Producer.
> >        1. FFA_MSG_WAIT ...
> >        2. FFA_RX_RELEASE.
> > "
> > 
> > Hope that helps, can dig deeper if there are any ambiguities around this.
> 
> Thanks Sudeep, but that also makes it sound like we need the RX_RELEASE
> even if we're not using the RX buffer per the input flags. :/
> 

Good spot, I had forgotten about the input flags that can avoid using the
buffer. I will see if we can improve the spec in that regards.

-- 
Regards,
Sudeep