Re: [PATCH 3/3] KVM: arm64: Raise KVM's reported debug architecture to v8.2

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oupton@google.com>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Andrew Jones <drjones@redhat.com>,
	Peter Shier <pshier@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Reiji Watanabe <reijiw@google.com>
Subject: Re: [PATCH 3/3] KVM: arm64: Raise KVM's reported debug architecture to v8.2
Date: Fri, 29 Oct 2021 12:31:16 +0100	[thread overview]
Message-ID: <87k0hw9iez.wl-maz@kernel.org> (raw)
In-Reply-To: <20211029003202.158161-4-oupton@google.com>

On Fri, 29 Oct 2021 01:32:02 +0100,
Oliver Upton <oupton@google.com> wrote:
> 
> The additions made to the Debug architecture between v8.0 and v8.2 are
> only applicable to external debug. KVM does not (and likely will never)
> support external debug, so KVM can proudly report support for v8.2 to
> its guests.
> 
> Raise the reported Debug architecture to v8.2. Additionally, v8.2 makes
> FEAT_DoubleLock optional. Even though KVM never supported it in the
> first place, report DoubleLock as not implemented now as the
> architecture permits it for v8.2.
> 
> Cc: Reiji Watanabe <reijiw@google.com>
> Cc: Ricardo Koller <ricarkol@google.com>
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 0840ae081290..f56ee5830d18 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1109,9 +1109,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  				 ARM64_FEATURE_MASK(ID_AA64ISAR1_GPI));
>  		break;
>  	case SYS_ID_AA64DFR0_EL1:
> -		/* Limit debug to ARMv8.0 */
> +		/* Limit debug to ARMv8.2 */
>  		val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_DEBUGVER);
> -		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_DEBUGVER), 6);
> +		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_DEBUGVER), 8);
> +
> +		/* Hide DoubleLock from guests */
> +		val &= ~ARM64_FEATURE_MASK(ID_AA64DFR0_DOUBLELOCK);
> +		val |= FIELD_PREP(ARM64_FEATURE_MASK(ID_AA64DFR0_DOUBLELOCK), 0CF);
> +

One issue with that is that this will break migration from an older
kernel (DFR0 will be different between source and destination).

You'll need a set_user handler and deal with it in a similar way to
CSV2/CSV3.

Thanks,

	M.

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

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