linux-kernel - Re: [PATCH 8/9] KVM: arm64: vgic-v3: Expose GICR

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Date:   Tue, 12 Jan 2021 17:48:34 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, drjones@...hat.com,
        james.morse@....com, julien.thierry.kdev@...il.com,
        suzuki.poulose@....com, shuah@...nel.org, pbonzini@...hat.com
Subject: Re: [PATCH 8/9] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for
 userspace

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...