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Subject: Re: [PATCH] KVM: arm64: Use the correct timer for accessing CNT
To:     Marc Zyngier <maz@kernel.org>
CC:     KarimAllah Ahmed <karahmed@amazon.de>,
        <linux-kernel@vger.kernel.org>, <kvmarm@lists.cs.columbia.edu>,
        <linux-arm-kernel@lists.infradead.org>
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 <7ed91b9b-e968-770c-28f9-0ca479359657@huawei.com>
 <a8b72d6c0a28e0554050e98d011f32d9@kernel.org>
From:   Zenghui Yu <yuzenghui@huawei.com>
Message-ID: <a9fd7e22-f46a-fd47-26ee-44d2d36783fd@huawei.com>
Date:   Mon, 16 Mar 2020 20:38:36 +0800
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Hi Marc,

On 2020/3/16 19:09, Marc Zyngier wrote:
> 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).

Yeah, I see. The physical timer registers are all ignored in
walk_one_sys_reg() and won't be exposed.

> 
> 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.

Thanks for the explanation.


Zenghui