linux-kernel - Re: [PATCH] KVM: arm64: selftest: Add standalone test checking for KVM's own UUID

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Message-ID: <86a54c9y19.wl-maz@kernel.org>
Date: Wed, 06 Aug 2025 18:10:10 +0100
From: Marc Zyngier <maz@...nel.org>
To: Andrew Jones <ajones@...tanamicro.com>
Cc: kvmarm@...ts.linux.dev,
	kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Will Deacon <will@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	jackabt@...zon.com
Subject: Re: [PATCH] KVM: arm64: selftest: Add standalone test checking for KVM's own UUID

On Tue, 22 Jul 2025 10:18:10 +0100,
Andrew Jones <ajones@...tanamicro.com> wrote:
> 
> Hi Marc,
> 
> On Mon, Jul 21, 2025 at 04:51:36PM +0100, Marc Zyngier wrote:
> > Tinkering with UUIDs is a perilious task, and the KVM UUID gets
> > broken at times. In order to spot this early enough, add a selftest
> > that will shout if the expected value isn't found.
> > 
> > Signed-off-by: Marc Zyngier <maz@...nel.org>
> > Link: https://lore.kernel.org/r/20250721130558.50823-1-jackabt.amazon@gmail.com
> > ---
> >  tools/testing/selftests/kvm/Makefile.kvm     |  1 +
> >  tools/testing/selftests/kvm/arm64/kvm-uuid.c | 67 ++++++++++++++++++++
> >  2 files changed, 68 insertions(+)
> >  create mode 100644 tools/testing/selftests/kvm/arm64/kvm-uuid.c
> > 
> > diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> > index ce817a975e50a..e1eb1ba238a2a 100644
> > --- a/tools/testing/selftests/kvm/Makefile.kvm
> > +++ b/tools/testing/selftests/kvm/Makefile.kvm
> > @@ -167,6 +167,7 @@ TEST_GEN_PROGS_arm64 += arm64/vgic_irq
> >  TEST_GEN_PROGS_arm64 += arm64/vgic_lpi_stress
> >  TEST_GEN_PROGS_arm64 += arm64/vpmu_counter_access
> >  TEST_GEN_PROGS_arm64 += arm64/no-vgic-v3
> > +TEST_GEN_PROGS_arm64 += arm64/kvm-uuid
> >  TEST_GEN_PROGS_arm64 += access_tracking_perf_test
> >  TEST_GEN_PROGS_arm64 += arch_timer
> >  TEST_GEN_PROGS_arm64 += coalesced_io_test
> > diff --git a/tools/testing/selftests/kvm/arm64/kvm-uuid.c b/tools/testing/selftests/kvm/arm64/kvm-uuid.c
> > new file mode 100644
> > index 0000000000000..89d9c8b182ae5
> > --- /dev/null
> > +++ b/tools/testing/selftests/kvm/arm64/kvm-uuid.c
> > @@ -0,0 +1,67 @@
> > +#include <errno.h>
> > +#include <linux/arm-smccc.h>
> > +#include <asm/kvm.h>
> > +#include <kvm_util.h>
> > +
> > +#include "processor.h"
> > +
> > +/*
> > + * Do NOT redefine these constants, or try to replace them with some
> > + * "common" version. They are hardcoded here to detect any potential
> > + * breakage happening in the rest of the kernel.
> > + *
> > + * KVM UID value: 28b46fb6-2ec5-11e9-a9ca-4b564d003a74
> > + */
> > +#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0	0xb66fb428U
> > +#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1	0xe911c52eU
> > +#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2	0x564bcaa9U
> > +#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3	0x743a004dU
> > +
> > +static void guest_code(void)
> > +{
> > +	struct arm_smccc_res res = {};
> > +
> > +	smccc_hvc(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, 0, 0, 0, 0, 0, 0, 0, &res);
> > +
> > +	__GUEST_ASSERT(res.a0 != SMCCC_RET_NOT_SUPPORTED, "a0 = %lx\n", res.a0);
> 
> Should this check res.a0 == SMCCC_RET_SUCCESS instead?

Sorry for the delay, but now that I'm awake, it is obvious this
doesn't work at all. A0-A3 contain the full UID.

While checking for A0 != NOT_SUPPORTED is valid, checking for
A0 == SUCCESS cannot work on KVM (we'd get 0xb66fb428 instead of 0).

So actually, this assertion is meaningless, and I'm dropping it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.