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Re: [RFC PATCH v2 0/5] target/arm/kvm: Adjust virtual time


From: Marc Zyngier
Subject: Re: [RFC PATCH v2 0/5] target/arm/kvm: Adjust virtual time
Date: Mon, 16 Dec 2019 16:18:23 +0000
User-agent: Roundcube Webmail/0.7.2

On 2019-12-16 15:33, Peter Maydell wrote:
On Thu, 12 Dec 2019 at 17:33, Andrew Jones <address@hidden> 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)?

Thanks,

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



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