Re: [Qemu-arm] [PATCH v4 03/21] target/arm: Reorganize PMCCNTR accesses

[Peter Maydell]

On 17 April 2018 at 21:37, Aaron Lindsay <address@hidden> wrote:
> pmccntr_read and pmccntr_write contained duplicate code that was already
> being handled by pmccntr_sync. Consolidate the duplicated code into two
> functions: pmccntr_op_start and pmccntr_op_finish. Add a companion to
> c15_ccnt in CPUARMState so that we can simultaneously save both the
> architectural register value and the last underlying cycle count - this
> ensure time isn't lost and will also allow us to access the 'old'
> architectural register value in order to detect overflows in later
> patches.
>
> Signed-off-by: Aaron Lindsay <address@hidden>
> ---
>  target/arm/cpu.h    |  28 ++++++++++-----
>  target/arm/helper.c | 100 
> ++++++++++++++++++++++++++++------------------------
>  2 files changed, 73 insertions(+), 55 deletions(-)
>
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index 19a0c03..04041db 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -454,10 +454,20 @@ typedef struct CPUARMState {
>          uint64_t oslsr_el1; /* OS Lock Status */
>          uint64_t mdcr_el2;
>          uint64_t mdcr_el3;
> -        /* If the counter is enabled, this stores the last time the counter
> -         * was reset. Otherwise it stores the counter value
> +        /* Stores the architectural value of the counter *the last time it 
> was
> +         * updated* by pmccntr_op_start. Accesses should always be surrounded
> +         * by pmccntr_op_start/pmccntr_op_finish to guarantee the latest
> +         * architecturally-corect value is being read/set.
>           */
>          uint64_t c15_ccnt;
> +        /* Stores the delta between the architectural value and the 
> underlying
> +         * cycle count during normal operation. It is used to update c15_ccnt
> +         * to be the correct architectural value before accesses. During
> +         * accesses, c15_ccnt_delta contains the underlying count being used
> +         * for the access, after which it reverts to the delta value in
> +         * pmccntr_op_finish.
> +         */
> +        uint64_t c15_ccnt_delta;

So the key question here is: how does this work for VM migration?

thanks
-- PMM