Re: [PATCH] xen/arm: Add Cortex-A73 erratum 858921 workaround

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi,

> On 9 Nov 2020, at 08:21, Penny Zheng <Penny.Zheng@xxxxxxx> wrote:
> 
> CNTVCT_EL0 or CNTPCT_EL0 counter read in Cortex-A73 (all versions)
> might return a wrong value when the counter crosses a 32bit boundary.
> 
> Until now, there is no case for Xen itself to access CNTVCT_EL0,
> and it also should be the Guest OS's responsibility to deal with
> this part.
> 
> But for CNTPCT, there exists several cases in Xen involving reading
> CNTPCT, so a possible workaround is that performing the read twice,
> and to return one or the other depending on whether a transition has
> taken place.
> 
> Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Regards
Bertrand

> ---
> docs/misc/arm/silicon-errata.txt |  1 +
> xen/arch/arm/Kconfig             | 18 ++++++++++++++++++
> xen/arch/arm/cpuerrata.c         |  8 ++++++++
> xen/arch/arm/vtimer.c            |  2 +-
> xen/include/asm-arm/cpuerrata.h  |  2 ++
> xen/include/asm-arm/cpufeature.h |  3 ++-
> xen/include/asm-arm/time.h       | 22 +++++++++++++++++++++-
> 7 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/misc/arm/silicon-errata.txt 
> b/docs/misc/arm/silicon-errata.txt
> index 1f18a9df58..552c4151d3 100644
> --- a/docs/misc/arm/silicon-errata.txt
> +++ b/docs/misc/arm/silicon-errata.txt
> @@ -51,6 +51,7 @@ stable hypervisors.
> | ARM            | Cortex-A57      | #1319537        | N/A                    
>  |
> | ARM            | Cortex-A72      | #1319367        | N/A                    
>  |
> | ARM            | Cortex-A72      | #853709         | N/A                    
>  |
> +| ARM            | Cortex-A73      | #858921         | ARM_ERRATUM_858921    
>   |
> | ARM            | Cortex-A76      | #1165522        | N/A                    
>  |
> | ARM            | Neoverse-N1     | #1165522        | N/A
> | ARM            | MMU-500         | #842869         | N/A                    
>  |
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index 2777388265..f938dd21bd 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -226,6 +226,24 @@ config ARM64_ERRATUM_834220
> 
>         If unsure, say Y.
> 
> +config ARM_ERRATUM_858921
> +     bool "Cortex-A73: 858921: Possible wrong read value for CNTVCT or 
> CNTPCT"
> +     default y
> +     help
> +       This option adds an alternative code sequence to work around ARM
> +       erratum 858921 on Cortex-A73 (all versions).
> +
> +       Affected Cortex-A73 might return wrong read value for CNTVCT or CNTPCT
> +       when the counter crosses a 32bit boundary.
> +
> +       The workaround involves performing the read twice, and to return
> +       one or the other value depending on whether a transition has taken 
> place.
> +       Please note that this does not necessarily enable the workaround,
> +       as it depends on the alternative framework, which will only patch
> +       the kernel if an affected CPU is detected.
> +
> +       If unsure, say Y.
> +
> endmenu
> 
> config ARM64_HARDEN_BRANCH_PREDICTOR
> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
> index 6731d873e8..567911d380 100644
> --- a/xen/arch/arm/cpuerrata.c
> +++ b/xen/arch/arm/cpuerrata.c
> @@ -469,6 +469,14 @@ static const struct arm_cpu_capabilities arm_errata[] = {
>         .capability = ARM_SSBD,
>         .matches = has_ssbd_mitigation,
>     },
> +#endif
> +#ifdef CONFIG_ARM_ERRATUM_858921
> +    {
> +        /* Cortex-A73 (all versions) */
> +        .desc = "ARM erratum 858921",
> +        .capability = ARM_WORKAROUND_858921,
> +        MIDR_ALL_VERSIONS(MIDR_CORTEX_A73),
> +    },
> #endif
>     {
>         /* Neoverse r0p0 - r2p0 */
> diff --git a/xen/arch/arm/vtimer.c b/xen/arch/arm/vtimer.c
> index 6d39fc944f..c2b27915c6 100644
> --- a/xen/arch/arm/vtimer.c
> +++ b/xen/arch/arm/vtimer.c
> @@ -62,7 +62,7 @@ static void virt_timer_expired(void *data)
> 
> int domain_vtimer_init(struct domain *d, struct xen_arch_domainconfig *config)
> {
> -    d->arch.virt_timer_base.offset = READ_SYSREG64(CNTPCT_EL0);
> +    d->arch.virt_timer_base.offset = get_cycles();
>     d->time_offset.seconds = ticks_to_ns(d->arch.virt_timer_base.offset - 
> boot_count);
>     do_div(d->time_offset.seconds, 1000000000);
> 
> diff --git a/xen/include/asm-arm/cpuerrata.h b/xen/include/asm-arm/cpuerrata.h
> index 88ef3ca934..8d7e7b9375 100644
> --- a/xen/include/asm-arm/cpuerrata.h
> +++ b/xen/include/asm-arm/cpuerrata.h
> @@ -28,6 +28,8 @@ static inline bool check_workaround_##erratum(void)         
>     \
> CHECK_WORKAROUND_HELPER(766422, ARM32_WORKAROUND_766422, CONFIG_ARM_32)
> CHECK_WORKAROUND_HELPER(834220, ARM64_WORKAROUND_834220, CONFIG_ARM_64)
> CHECK_WORKAROUND_HELPER(ssbd, ARM_SSBD, CONFIG_ARM_SSBD)
> +CHECK_WORKAROUND_HELPER(858921, ARM_WORKAROUND_858921,
> +                        CONFIG_ARM_ERRATUM_858921)
> 
> #undef CHECK_WORKAROUND_HELPER
> 
> diff --git a/xen/include/asm-arm/cpufeature.h 
> b/xen/include/asm-arm/cpufeature.h
> index 10878ead8a..016a9fe203 100644
> --- a/xen/include/asm-arm/cpufeature.h
> +++ b/xen/include/asm-arm/cpufeature.h
> @@ -45,8 +45,9 @@
> #define ARM_SSBD 7
> #define ARM_SMCCC_1_1 8
> #define ARM64_WORKAROUND_AT_SPECULATE 9
> +#define ARM_WORKAROUND_858921 10
> 
> -#define ARM_NCAPS           10
> +#define ARM_NCAPS           11
> 
> #ifndef __ASSEMBLY__
> 
> diff --git a/xen/include/asm-arm/time.h b/xen/include/asm-arm/time.h
> index 9cb6f9b0b4..1b2c13614b 100644
> --- a/xen/include/asm-arm/time.h
> +++ b/xen/include/asm-arm/time.h
> @@ -3,6 +3,7 @@
> 
> #include <asm/sysregs.h>
> #include <asm/system.h>
> +#include <asm/cpuerrata.h>
> 
> #define DT_MATCH_TIMER                      \
>     DT_MATCH_COMPATIBLE("arm,armv7-timer"), \
> @@ -13,7 +14,26 @@ typedef uint64_t cycles_t;
> static inline cycles_t get_cycles (void)
> {
>         isb();
> -        return READ_SYSREG64(CNTPCT_EL0);
> +        /*
> +         * ARM_WORKAROUND_858921: Cortex-A73 (all versions) counter read
> +         * can return a wrong value when the counter crosses a 32bit 
> boundary.
> +         */
> +        if ( !check_workaround_858921() )
> +            return READ_SYSREG64(CNTPCT_EL0);
> +        else
> +        {
> +            /*
> +             * A recommended workaround for erratum 858921 is to:
> +             *  1- Read twice CNTPCT.
> +             *  2- Compare bit[32] of the two read values.
> +             *      - If bit[32] is different, keep the old value.
> +             *      - If bit[32] is the same, keep the new value.
> +             */
> +            cycles_t old, new;
> +            old = READ_SYSREG64(CNTPCT_EL0);
> +            new = READ_SYSREG64(CNTPCT_EL0);
> +            return (((old ^ new) >> 32) & 1) ? old : new;
> +        }
> }
> 
> /* List of timer's IRQ */
> -- 
> 2.25.1
>