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

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 16 Dec 2019 at 15:33, Peter Maydell <peter.maydell@linaro.org> wrote:
> 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.)
>
> QEMU does assume that the kernel's ID register values match
> the hardware for sysregs in some ways -- we use this when we
> construct our mapping from KVM register IDs as returned by
> KVM_GET_REG_LIST to QEMU cpreg definitions and thus CPUState
> struct fields. I *think* that in this case the only visible
> effect will be that gdbstub will show you the CNT value
> if you ask it to print the value of the CVAL sysreg.

...perhaps we should work around this kernel bug in the
kvm_to_cpreg_id() and cpreg_to_kvm_id() functions. (Need
to think through/test whether that would break migration.)

thanks
-- PMM