linux-kernel - Re: [PATCH] KVM: arm64: Fix error checking for FFA

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Message-ID: <CACF_fqmWwtuF_7+OJ2rUPvDJzs4ubJO6XCf6N9cW2-AABSx_NQ@mail.gmail.com>
Date: Mon, 24 Nov 2025 12:49:08 +0100
From: Kornel Dulęba <korneld@...gle.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oupton@...nel.org>, Joey Gouly <joey.gouly@....com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Bartlomiej Grzesik <bgrzesik@...gle.com>, Tomasz Nowicki <tnowicki@...gle.com>, 
	Sebastian Ene <sebastianene@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Fix error checking for FFA_VERSION

On Sat, Nov 22, 2025 at 12:36 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On Fri, 14 Nov 2025 11:11:53 +0000,
> "=?utf-8?q?Kornel_Dul=C4=99ba?=" <korneld@...gle.com> wrote:
> >
> > According to section 13.2 of the DEN0077 FF-A specification, when
> > firmware does not support the requested version, it should reply with
> > FFA_RET_NOT_SUPPORTED(-1). Table 13.6 specifies the type of the error
> > code as int32.
> > Currently, the error checking logic compares the unsigned long return
> > value it got from the SMC layer, against a "-1" literal. This fails due
> > to a type mismatch: the literal is extended to 64 bits, whereas the
> > register contains only 32 bits of ones(0x00000000ffffffff).
> > Consequently, hyp_ffa_init misinterprets the "-1" return value as an
> > invalid FF-A version. This prevents pKVM initialization on devices where
> > FF-A is not supported in firmware.
>
> Is this statement accurate? I regularly boot KVM in protected mode in
> environments that really cannot be suspected of implementing FF-A
> (there is no EL3 to start with). And yet I don't see any failure of
> the sort.
>
> Please clarify the circumstances this is triggered.

I do have EL3 enabled, but the FF-A itself is not implemented.

Without this change kvm initialization fails with the following:

[    0.946776][    T1] kvm [1]: nv: 554 coarse grained trap handlers
[    0.952880][    T1] kvm [1]: nv: 669 fine grained trap handlers
[    0.958813][    T1] kvm [1]: IPA Size Limit: 44 bits
(...)
[    1.034089][    T1] kvm [1]: Failed to init hyp memory protection
[    1.041213][    T1] kvm [1]: error initializing Hyp mode: -95

I managed to narrow this down to the FFA version check by examining
all of the places in kvm initialization logic where -EOPNOTSUPP is
returned. Since printing anything in this part of the code is somewhat
problematic I replaced “return -EOPNOTSUPP” with “return res.s0” to
examine the problematic register value:

[1.041229][    T1] kvm [1]: error initializing Hyp mode: -1

Note that the return code itself is cast to int before being printed.
Then a colleague of mine recommended looking into the arm_ffa driver.
(“drivers/firmware/arm_ffa/driver.c”) There I found that in the
ffa_version_check function, the return value from the SMC call is cast
to s32 before being checked for errors.
I did the same in the kvm initialization logic, which is how this
patch was created. Furthermore I also examined the FF-A
specification(DEN0077), where the error code value type is specified
as int32.
With this change applied I can now see that kvm is up and running:

[    0.946839][    T1] kvm [1]: nv: 554 coarse grained trap handlers
[    0.952940][    T1] kvm [1]: nv: 669 fine grained trap handlers
[    0.958867][    T1] kvm [1]: IPA Size Limit: 44 bits
(...)
[    1.061717][    T1] kvm [1]: Protected hVHE mode initialized successfully

The /dev/kvm file is also there.