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To: Andrew Jones <drjones@redhat.com>
Subject: Re: [RFC PATCH v2 0/5] target/arm/kvm: Adjust virtual time
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Date: Mon, 16 Dec 2019 17:05:06 +0000
From: Marc Zyngier <maz@kernel.org>
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On 2019-12-16 16:59, Andrew Jones wrote:
> On Mon, Dec 16, 2019 at 04:18:23PM +0000, Marc Zyngier wrote:
>> On 2019-12-16 15:33, Peter Maydell wrote:
>> > On Thu, 12 Dec 2019 at 17:33, Andrew Jones <drjones@redhat.com> 
>> wrote:
>> >
>> > > Userspace that wants to set KVM_REG_ARM_TIMER_CNT should beware 
>> that
>> > > the KVM register ID is not correct.  This cannot be fixed 
>> because
>> > > it's
>> > > UAPI and if the UAPI headers are used then it can't be a 
>> problem.
>> > > However, if a userspace attempts to create the ID themselves 
>> from
>> > > the
>> > > register's specification, then they will get 
>> KVM_REG_ARM_TIMER_CVAL
>> > > instead, as the _CNT and _CVAL definitions have their register
>> > > parameters swapped.
>> >
>> > So, to be clear, you mean that:
>> >
>> > (1) the kernel headers say:
>> >
>> > /* EL0 Virtual Timer Registers */
>> > #define KVM_REG_ARM_TIMER_CTL           ARM64_SYS_REG(3, 3, 14, 3, 
>> 1)
>> > #define KVM_REG_ARM_TIMER_CNT           ARM64_SYS_REG(3, 3, 14, 3, 
>> 2)
>> > #define KVM_REG_ARM_TIMER_CVAL          ARM64_SYS_REG(3, 3, 14, 0, 
>> 2)
>> >
>> > (2) some of the RHSes of these are wrong
>> >
>> > (3) but the kernel internally is using the same 'wrong' value, so
>> > userspace also needs to use that value, ie trust the #defined name
>> > rather than manufacturing one ?
>> >
>> > That's awkward. I think it would be worth at least having a kernel
>> > patch to add a comment clearly documenting this bug.
>> >
>> > (This error seems to only be in the 64-bit ABI, not 32-bit.)
>>
>> Yeah, this is pretty bad. I wonder how we managed not to notice
>> this for so long... :-(.
>>
>> Andrew, could you please write a patch documenting this (both in
>> the UAPI headers and in the documentation)?
>>
>
> Will do. I'll try to get to it this week.

Thanks a lot.

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