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Date:   Mon, 16 Mar 2020 11:09:03 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     Zenghui Yu <yuzenghui@huawei.com>
Cc:     KarimAllah Ahmed <karahmed@amazon.de>,
        linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] KVM: arm64: Use the correct timer for accessing CNT
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Hi Zenghui,

On 2020-03-16 10:49, Zenghui Yu wrote:
> Hi,
> 
> On 2020/3/16 17:39, KarimAllah Ahmed wrote:
>> Use the physical timer object when reading the physical timer counter
>> instead of using the virtual timer object. This is only visible when
>> reading it from user-space as kvm_arm_timer_get_reg() is only executed 
>> on
>> the get register patch from user-space.
> 
> s/patch/path/
> 
> I think the physical counter hasn't yet been accessed by the current
> userspace, wrong?

I don't think userspace can access it, as the ONE_REG API only exposes 
the virtual
timer so far, and userspace is much better off just reading the counter 
directly
(it has access to the virtual counter, and the guarantee that cntvoff is 
0 in this
context).

But as we move towards a situation where we can save/restore the 
physical timer
just like the virtual one, we're going to use this path and hit this 
bug.

> 
>> 
>> Cc: Marc Zyngier <maz@kernel.org>
>> Cc: James Morse <james.morse@arm.com>
>> Cc: Julien Thierry <julien.thierry.kdev@gmail.com>
>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: kvmarm@lists.cs.columbia.edu
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> 
> Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
> 
> And this might also deserve:
> 
> Fixes: 84135d3d18da ("KVM: arm/arm64: consolidate arch timer trap 
> handlers")

Indeed. Thanks,

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