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From: Marc Zyngier <maz@kernel.org>
To: Alexandru Elisei <alexandru.elisei@arm.com>
Cc: Eric Auger <eric.auger@redhat.com>,
	eric.auger.pro@gmail.com, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
	drjones@redhat.com, james.morse@arm.com,
	julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com,
	shuah@kernel.org, pbonzini@redhat.com
Subject: Re: [PATCH 8/9] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for userspace
Date: Tue, 12 Jan 2021 17:48:34 +0000	[thread overview]
Message-ID: <1a06be3153927f1051fcbc87f0e52e98@kernel.org> (raw)
In-Reply-To: <fe0a3415-0c7b-be13-6438-89e82fe4c281@arm.com>

On 2021-01-12 17:28, Alexandru Elisei wrote:
> Hi Eric,
> 
> On 12/12/20 6:50 PM, Eric Auger wrote:
>> Commit 23bde34771f1 ("KVM: arm64: vgic-v3: Drop the
>> reporting of GICR_TYPER.Last for userspace") temporarily fixed
>> a bug identified when attempting to access the GICR_TYPER
>> register before the redistributor region setting but dropped
>> the support of the LAST bit. This patch restores its
>> support (if the redistributor region was set) while keeping the
>> code safe.
> 
> If I understand your patch correctly, it is possible for the 
> GICR_TYPER.Last bit
> to be transiently 1 if the register is accessed before all the 
> redistributors
> regions have been configured.
> 
> Arm IHI 0069F states that accesses to the GICR_TYPER register are RO. I 
> haven't
> found exactly what RO means (please point me to the definition if you 
> find it in
> the architecture!), but I assume it means read-only and I'm not sure 
> how correct
> (from an architectural point of view) it is for two subsequent reads of 
> this
> register to return different values. Maybe Marc can shed some light on 
> this.

RO = Read-Only indeed. Not sure that's documented anywhere in the 
architecture,
but this is enough of a well known acronym that even the ARM ARM doesn't 
feel
the need to invent a new definition for it.

As for your concern, I don't think it is a problem to return different 
values
if the HW has changed in between. This may need to be documented though.

Thanks,

        M.
-- 
Jazz is not dead. It just smells funny...

  reply	other threads:[~2021-01-12 17:49 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-12 18:50 [PATCH 0/9] KVM/ARM: Some vgic fixes and init sequence KVM selftests Eric Auger
2020-12-12 18:50 ` [PATCH 1/9] KVM: arm64: vgic-v3: Fix some error codes when setting RDIST base Eric Auger
2021-01-06 16:32   ` Alexandru Elisei
2021-01-14 10:02     ` Auger Eric
2020-12-12 18:50 ` [PATCH 2/9] KVM: arm64: Fix KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION read Eric Auger
2021-01-06 17:12   ` Alexandru Elisei
2021-01-13 17:18     ` Auger Eric
2020-12-12 18:50 ` [PATCH 3/9] KVM: arm64: vgic-v3: Fix error handling in vgic_v3_set_redist_base() Eric Auger
2020-12-28 15:35   ` Marc Zyngier
2021-01-13 17:18     ` Auger Eric
2020-12-12 18:50 ` [PATCH 4/9] KVM: arm/arm64: vgic: Reset base address on kvm_vgic_dist_destroy() Eric Auger
2020-12-28 15:41   ` Marc Zyngier
2021-01-13 17:18     ` Auger Eric
2020-12-12 18:50 ` [PATCH 5/9] KVM: arm: move has_run_once after the map_resources Eric Auger
2021-01-12 14:55   ` Alexandru Elisei
2021-01-14 10:02     ` Auger Eric
2021-01-20 15:56       ` Alexandru Elisei
2021-03-12 17:27         ` Auger Eric
2020-12-12 18:50 ` [PATCH 6/9] docs: kvm: devices/arm-vgic-v3: enhance KVM_DEV_ARM_VGIC_CTRL_INIT doc Eric Auger
2021-01-12 15:39   ` Alexandru Elisei
2021-01-13 17:18     ` Auger Eric
2020-12-12 18:50 ` [PATCH 7/9] KVM: arm64: Simplify argument passing to vgic_uaccess_[read|write] Eric Auger
2021-01-12 16:04   ` Alexandru Elisei
2021-01-12 16:16     ` Alexandru Elisei
2021-01-13 17:18       ` Auger Eric
2020-12-12 18:50 ` [PATCH 8/9] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for userspace Eric Auger
2021-01-12 17:02   ` Alexandru Elisei
2021-01-14 10:16     ` Auger Eric
2021-01-20 16:13       ` Alexandru Elisei
2021-03-12 17:26         ` Auger Eric
2021-01-12 17:28   ` Alexandru Elisei
2021-01-12 17:48     ` Marc Zyngier [this message]
2020-12-12 18:50 ` [PATCH 9/9] KVM: selftests: aarch64/vgic-v3 init sequence tests Eric Auger

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